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Sign return addresses in the hardened Linux arm64 lane with the ahead-of-time compiler - #968
Follows #945, which left the lanes with the ahead-of-time compiler out: ENABLE(RETURN_ADDRESS_SIGNING) with ENABLE(AOT) was an #error. This takes the #error away and gives bun-webkit-linux-arm64-lto-hardened-aot the flags that -lto-hardened has. No other lane changes.
what the ahead-of-time compiler generates: functions, stubs, regular expressions, thunks, WebAssembly
does not
An image does not sign, so that it is the same whether or not the compiler that makes it has the switch, and runs with a bun that is compiled this way and with one that is not. The instructions are hints, so code that has them signs on any processor with pointer authentication, whatever run time it is loaded by. And the run time does more with the return address of a compiled frame than read it: it writes one (setReturnPC() in AOTUnwinding.cpp) and compares it with the address of a stub as it is (AOTCoroutineTable.cpp, AOTJobs.cpp). All of that stays as it is.
A frame without a signature among frames with one is nothing new: that is what a frame of a library compiled without the flag is. Each function checks only what it signed itself, and whatever walks the stack takes off a signature that may be there.
How
AssemblerPolicy::signsReturnAddresses, false in forImage(). Whatever goes into an image is assembled with that policy already, for its relocations. MacroAssemblerARM64 asks it in the eight functions that emit a signing instruction (tagReturnAddress(), untagReturnAddress(), both forms of tagPtr() and untagPtr(), removePtrTag(), validateUntaggedPtr(), ret()). None of the places in aot/ that emit a prologue or a return changes.
The run time takes the signature off what it reads. It reads the return address in the frame record of its own operations, which are C++ and sign, and those of whatever else is on the stack. The calls of removeCodePtrTag() with a saved return address for an argument are removeReturnAddressSignature() now, which is removeCodePtrTag() everywhere else: 33 in aot/runtime, six of which came with #942, and two in StackVisitor.cpp. Ten of the 33 had the word from rawReturnPC() and are returnPCForInspection(), which is the two together. The one that is left in aot/ has an address that compiled code passes as an argument.
One of the two in StackVisitor.cpp is isCalledByCompiledCode(), new in #942, which compares the word with the bounds of the image's code. With a signature in it the address is in no image, and the visitor took compiled frames for the interpreter's.
JSPI leaves the return addresses of an image's frames as they are.EvacuatedStackSlice signs the return address of every frame anew when it takes the frames off the stack and when it puts them back. What WebAssembly code of an image has saved has no signature to authenticate. The callee of the frame says whether its code is in an image. The word itself does not: a signature may be all zeros. The frame that implant() returns from is signed whatever its callee. With SlicingStrategy::Slab it is the frame of the function that suspends, which is no code of an image, and a RELEASE_ASSERT says so now.
What lets Air emit the frame does not sign. With frame.emitsFrame a path that needs no frame sets none up, so it has signed nothing when it gets to ret(). Only the ahead-of-time compiler sets that, with the policy of an image, and Air::generate() asserts it now.
Ten returns in LowLevelInterpreterAOT.asm are of thunks that make no frame (_llint_aot_char_code_at_thunk and its like), so they have not signed anything. offlineasm's ret authenticates. They return with returnWithoutFrame().
In the tests: the two landings of StubRunner and CalleeThatReadsItsClosure end in ret() without a frame and are not code of an image, so they begin with tagReturnAddress(). A third landing came with main in the last merge. It pops what a stub of an image saved, so it signs behind the epilogue. testair sets emitsFrame with the policy of the JIT, so its calls of generate() go through a function that takes the signing off with it. AOTOperationScopeTests.cpp and AOTSavedFrameTests.cpp had five more removeCodePtrTag().
Also here: a comment of #945 in wtf/PtrTag.h said that distributions compile the C library with the flag. I have no evidence for that, so it no longer says it.
How I tested it
On hardware: at b2c96a0 and at 3e99432 the test jobs got arm64 runners that have pointer authentication, and print so. Not all of that pool do: earlier the same day the jobs of #965 got runners without, and warned about it. In Test bun-webkit-linux-arm64-lto-hardened-aot, the lane of this PR, both times:
testaot
8652 passed, 0 failed, 42 todo. That is every one of the 5708 .ir tests and the 2986 native tests that the program has
testmasm, testair, testb3
pass
JSTests, LayoutTests/js and the rest of run-javascriptcore-tests
0 failures
So the 13 native tests that I cannot run, below, pass there with pointer authentication, and so do webassembly-instance/suspends-and-goes-on and the one that is too slow for the emulator. All 85 checks pass at both commits. 16de3d0, the head, has main at f25de97 in it and what the second review found, below.
In Bun's CI, on machines that print paca: oven-sh/bun#47554 (a draft, not for merging) built Bun for Linux arm64 twice at the same commit, with the compiler in and hardened. One links the -lto-hardened-aot lane of 3e99432 and is compiled with -mbranch-protection=pac-ret itself (C, C++ and Rust). The other, the control, signs nothing and links the same lane of #965 at 122e5da, which is main at 4d45755, like 3e99432, with changes that are off in that lane. Build 136636:
1: a test of that draft, which expected more than 100 PACIBSP
No hardened build with the compiler is in Bun's CI otherwise, which is why so many fail. At least 137 of the 373 say "this build of Bun only runs compiled code that is part of the executable itself": a hardened Bun refuses bun test --aot and bun run --aot. I have not looked at each of the others, only that the two lists are the same.
Of the 935 test files about the compiler, 363 are among the 373 and 572 are not. test/aot/engine/native-1 to -4, which are testaot native linked with Bun's table, so with the tests that are LEFT OUT here, pass. native-6 and ir-1, -2, -4 and -6 fail in both.
One of the 20 shards ran out of time in both, so 7214 of 7481 entries were reached in each.
A test that passes prints nothing there, so I have the counts of the executable that signs and not those of the control.
Under an emulator: everything below is qemu-aarch64, with -cpu neoverse-n2, which has pointer authentication and faults where it fails, and -cpu neoverse-n1, which has none. The builds have the lane's flags but for LTO.
What it was like with nothing but the #error taken away: of the 153 .ir tests that run a compiled program in the shell, 152 passed without pointer authentication and 3 with it.
The native tests that fail. One with pointer authentication alone: valueSetsOfBothFormsAgreeWithPlainSets "printed nothing for 100 seconds". Alone and with --single-process it passes in 144 seconds, against 80 without the switch: the emulator computes every signature in software.
16 fail on both. Two of them do now and then wherever I run them, and pass alone: collector/helpers/small-heap-starts-none and startup/main-thread-stack-ends-above-its-first-frame. child-process/trap-is-reported is new on that list. It has been in the tree since #942 and passed on the machine that I had then, and what it looks at is how the trap of a child is reported, which the emulator does. The other 13 say nothing about this PR, because they fail in my build whatever the processor, and without the switch too: stubs/call-method-by-id/closures-not-created and 12 of stubs/get-by-id, stubs/put-by-id and stubs/get-by-val. They put fake objects on the stack and depend on where those are, which is different under the emulator. They pass in the test job, above. Before that run I had only read: of the 20 places in aot/tests that declare an assembler, the three named above were the ones that return without a frame, a signature or the policy of an image.
What failed with pointer authentication alone on the way here, and passes now:
12 of stubs/* with SIGILL, where a landing of StubRunner authenticated what nothing had signed, and operation-scope/the-frame-record-holds-the-return-address.
webassembly-instance/suspends-and-goes-on, which is new here and suspends frames of an image's code: without the change to EvacuatedStack.cpp the shell ends with SIGILL.
After the merge with f25de97, stubs/frames-of-slow-paths-have-no-word-that-nobody-wrote, which came with it: its landing, above. Without the tagReturnAddress() it ends with SIGILL with pointer authentication and passes without.
The rest of this section is of 35a4d79, before the merges.
An image is the same. 26 images compiled by this build and by one without the switch are the same byte for byte. All eight places in MacroAssemblerARM64.h that emit a hint ask the policy. removePtrTag() has since 3e99432, whose message says that an image with a cross-instance tail call of WebAssembly differed until then. That was wrong: no function with a tail call goes into an image (UsesTailCalls in WasmOMGIRGenerator.cpp), so nothing assembled it with that policy. emitRestoreInstanceFrameIfNeeded(), its one caller, asserts now that its policy signs. One of them had 328 PACIBSP and 1895 AUTIBSP with nothing but the #error taken away, and has none now, with 1895 ret in both.
Ordinary JavaScript, because the thunks without a frame are entered by any code: JSTests/stress with this build's jsc, with pointer authentication.
mode
ran
exit with 0
authentication failures
default
6230
6065
1
--useJIT=0
6233
6014
1
eager (thresholds of 10 and 20, no concurrent JIT)
6233
6008
1
The 1 is bun-return-address-with-wrong-signature-is-not-returned-to.js, which must crash. My runner takes the options of a test's first runDefault or requireOptions line, gives it 90 seconds and looks at nothing but the exit code, so tests fail in it for want of their options or of time. 19 other tests ended with a signal in one row or another, 16 of which set --watchdog. I ran each again on this build and on one without the switch, 10 times or more. 18 end so on both, at like rates: the 15 of the default row exactly as often on the one as on the other.
The one that did not is early-return-from-builtin.js: 18 of 60 runs here, 1 of 60 without the switch, 0 of 60 here without pointer authentication. It is the time that it takes, not the signature. It runs for 0.9 seconds here and for 0.7 in the two others, because the emulator computes signatures in software, and its watchdog is set to 1 second. With other settings, of 30 runs each:
watchdog, ms
500
600
700
800
900
1000
1200
this build, with pointer authentication
30
30
30
15
14
11
1
without the switch
28
12
6
4
0
0
0
this build, without pointer authentication
17
16
7
1
1
0
0
So a watchdog that fires in the middle of this test ends it with a segmentation fault under the emulator whatever the build. I have not looked at why. These tests pass in the test job, on arm64 hardware.
Tools/Scripts/check-signed-return-addresses.py on the jsc of 16de3d0: 58440 functions sign, 19 are on the list.
What keeps it true
testAssemblerPolicy in testmasm, which this lane builds and runs: all eight emitters and ret() assembled with forImage() are one instruction, the ret, and with forJIT() they have eight hints. It looks at the bytes, so it needs no pointer authentication.
The RELEASE_ASSERT in Air::generate(): what lets Air emit the frame is assembled with a policy that does not sign. It needs no pointer authentication either.
webassembly-instance/suspends-and-goes-on, which tells only on a processor that has it, like every test that throws or takes a stack trace out of compiled code.
Tools/Scripts/check-signed-return-addresses.py, behind the link in the lane.
New, and it is a rule for everybody's changes to aot/: the same script reads the sources too, and fails where aot/ has a removeCodePtrTag( that is not on its list, which has the one in AOTInstance.cpp. It needs no pointer authentication. With one planted it fails and names the file.
Why the rule: a new removeCodePtrTag() on a saved return address compiles, and #942 brought eleven in a day. Whether the test job sees one depends on which runner it gets. What still does not keep it true: a saved return address that is read with neither function, and code of a test that returns without a frame, like the landing above. Those show only on a runner with pointer authentication.
What the review found
Three things that I had missed, all fixed above: JSPI, testair, and CalleeThatReadsItsClosure, which is in one of the 13 tests that I cannot run. And that testAssemblerPolicy would have failed for ARM64E, which signs with PACIBSP whatever the policy says. It is not compiled for ARM64E now. No lane is. And removePtrTag(), above.
A second review, of 3e99432, found no bug that can be reached. From it: returnPCForInspection(), the two new assertions, the source rule, testAssemblerPolicy calling every emitter, signedReturnAddresses on lto-hardened-aot-no-webassembly (built for x64 alone, so no lane's flags change), two calls in testair back to generate() where the wrapper did nothing, and the wrong sentence about tail calls.
Left as it is: in this lane the four trampolines that sign for a function of the interpreter go on to the ordinary prologues, so aotFunctionPrologue, which is faster, is not reached where an image runs. It costs speed and nothing else. Four of the strips in aot/runtime are of a word that its caller has stripped already. Both hardened lanes for arm64 have the signedReturnAddresses key of lanes.mjs now, and hardened-aot, which has not, is built for x64 alone. So the key could go, once #965 has landed too.
#965 gives -lto-hardened landing pads. The two do not touch the same lines of lanes.mjs. Landing pads are a key of their own there and an #error with ENABLE(AOT), so this lane does not get them along with the signing, whichever lands first.
…, but not by the code of an image
ENABLE(RETURN_ADDRESS_SIGNING) with ENABLE(AOT) was an #error. With nothing but the #error taken away, 3 of the
153 .ir tests that run a compiled program passed on a processor with pointer authentication.
What goes into an image does not sign: AssemblerPolicy::signsReturnAddresses is false in forImage(), and
MacroAssemblerARM64 asks it wherever it emits a signing instruction. So an image is the same whether or not
the compiler that makes it has the switch, and runs with a run time that has it and with one that has not.
The run time also writes the return address of a compiled frame and compares it with the address of a stub as
it is, and all of that stays as it is.
The run time is C++ and signs. What it reads out of a frame record may have a signature, its own operations'
above all, so the 27 calls of removeCodePtrTag() in aot/ that had a saved return address for an argument are
removeReturnAddressSignature().
Ten thunks in LowLevelInterpreterAOT.asm make no frame and sign nothing, and offlineasm's ret authenticates.
They return with returnWithoutFrame().
In the tests, the two landings of StubRunner end in ret() without a frame and are no code of an image, so they
begin with tagReturnAddress(). testAssemblerPolicy looks at the bytes of a function assembled with either
policy: no hint for an image, five for the JIT. It needs no processor with pointer authentication.
A comment in PtrTag.h said that distributions compile the C library with the flag, for which I have no evidence.
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Walkthrough
This change adds policy-controlled return-address signing for ARM64 AOT configurations. AOT runtime paths now remove return-address signatures before handling addresses. Stack relocation, tests, and hardened-lane descriptions are also updated.
Image policy disables return-address signing, while the default policy enables it. ARM64 assembler operations, AOT thunks, and B3 test generation follow the policy. The compile-time restriction on enabling AOT with return-address signing is removed. Lane descriptions are updated.
AOT frame, exception, and stack handling Source/JavaScriptCore/aot/runtime/*, Source/JavaScriptCore/interpreter/StackVisitor.cpp
AOT frame iteration, exception handling, stack traces, unwinding, and interpreter frame entry remove return-address signatures before lookups and comparisons.
Stack relocation and suspension handling Source/JavaScriptCore/runtime/EvacuatedStack.cpp, Source/JavaScriptCore/aot/tests/AOTWebAssemblyInstanceTests.cpp
Evacuated stack relocation distinguishes image Wasm return addresses from addresses that require signing. A WebAssembly suspension test checks successful resumption and output 42.
AOT return-address test helpers Source/JavaScriptCore/aot/tests/AOTOperationScopeTests.cpp, Source/JavaScriptCore/aot/tests/AOTStubRunner.h, Source/JavaScriptCore/aot/tests/AOTSavedFrameTests.cpp, Source/JavaScriptCore/aot/tests/AOTStubPropertyReadTests.cpp
Test helpers tag landing-stub return addresses and use signature removal for return-address comparisons and frame diagnostics.
Possibly related PRs
oven-sh/WebKit#945: Enables signed return addresses in the non-AOT hardened ARM64 lane and adds frame handling extended here to AOT.
oven-sh/WebKit#965: Defines the ARM64 hardened landing-pad configuration and its AOT exclusion, which this change removes.
oven-sh/WebKit#907: Adds the Linux ARM64 AOT hardened lane; this change adds signed return-address handling to that lane.
No actionable merge-blocking risk was established for the hardened AOT lane.
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Actionable comments posted: 1
Fix comments: Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @Source/JavaScriptCore/assembler/testmasm.cpp:
- Line 9447: Update MacroAssemblerARM64E::tagReturnAddress() and
untagReturnAddress() to emit signing or untagging instructions only when
policy().signsReturnAddresses is true; preserve their existing behavior when it
is true. Keep ARM64E included in the assertion in testAssemblerPolicy().
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Ten more calls of removeCodePtrTag() in aot/ with a saved return address for an argument, six in the run time
and four in AOTSavedFrameTests.cpp. They are removeReturnAddressSignature().
The one that is left, in AOTInstance.cpp, has the address at which a saved frame goes on. Compiled code passes
that to the operation as an argument, so nothing has signed it.
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Beyond the inline findings, I also checked the one place the run time writes a return address rather than reads one: setReturnPC() in AOTUnwinding.cpp (stackCaptureActionOfThrow) stores a compiled-code address into the frame of image code, which does not sign under forImage(), so an unsigned word there is what that frame's ret expects. The removeReturnAddressSignature() substitutions in aot/runtime are all on the read side (lookups and comparisons against image addresses), and returnWithoutFrame() only changes the thunks that enter without saving lr.
Extended reasoning...
The change lifts the ENABLE(RETURN_ADDRESS_SIGNING)+ENABLE(AOT) #error, adds AssemblerPolicy::signsReturnAddresses (false for images) gating the PAC hints in MacroAssemblerARM64, switches 28 removeCodePtrTag reads of saved return addresses to removeReturnAddressSignature, makes ten frameless LLInt AOT thunks return without AUTIBSP, and turns signing on for the arm64 lto-hardened-aot lane. It touches control-flow integrity on arm64 (pointer authentication), which is a security-relevant surface. Four inline findings stand (JSPI wasm frame re-signing, a test callee and a frameless Air test that still authenticate unsigned lr, and an ARM64E-only testmasm mismatch), and the author states no hardware with pointer authentication ran it, so a human look is needed; this note only records the write path and read-side substitutions that were examined and found consistent.
Findings marked 🟡 are optional suggestions and need no follow-up push.
MacroAssemblerARM64E signs whatever the policy says, with PACIBSP for one, which is a hint. So the check that
an image has none would have failed there. No lane is ARM64E.
…piled code called a frame
isCalledByCompiledCode(), which is new in #942, compares a saved return address with the bounds of the image's
code. With a signature in it the address is in no image, so the visitor took compiled frames for frames of the
interpreter: 70 native tests and 30 .ir tests that throw or take a stack trace crashed on a processor with
pointer authentication, and on no other.
EvacuatedStackSlice signs the return address of every frame anew when it takes the frames off the stack and
when it puts them back. WebAssembly code of an image does not sign, so authenticating what it saved failed: a
compiled program with a module in its image crashed at its first suspension on a processor with pointer
authentication.
The callee of the frame says whether its code is in an image. The word itself does not: a signature may be all
zeros. The frame that returns to the sentinel gets that address without a signature if it is an image's.
webassembly-instance/suspends-and-goes-on suspends two frames of an image's code. Without this the shell ends
with SIGILL under qemu-aarch64 -cpu neoverse-n2.
Like the two landings of StubRunner, it is no code of an image. Of the 20 assemblers in aot/tests it was the
only one left that returns without a frame, a signature or the policy of an image.
With frame.emitsFrame a path that needs no frame sets none up, so it has signed nothing when it gets to ret(),
which authenticates. Only the ahead-of-time compiler sets that, and it assembles with the policy of an image.
generate() asserts it now, on any processor.
testair sets it too, with the policy of the JIT, and would have crashed on a processor with pointer
authentication. Its five calls of generate() go through generateFollowingFramePolicy().
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
The commits since my last pass address the four threads I opened earlier (JSPI re-signing in EvacuatedStack.cpp, the test callee in AOTStubPropertyReadTests.cpp, the frame-less Air path in AirGenerate.cpp/testair.cpp, and the ARM64E guard in testmasm.cpp); beyond the one inline note from this run, I also checked emitRestoreInstanceFrameIfNeeded in WasmIRGeneratorHelpers.h:254, where removePtrTag() still emits XPACLRI under the image policy — harmless at run time (stripping an unsigned word is a no-op), though an image whose wasm module makes a cross-instance tail call is not byte-identical between a compiler with the switch and one without, contrary to the description's claim.
Extended reasoning...
The change enables ENABLE(RETURN_ADDRESS_SIGNING) with ENABLE(AOT) for the arm64 lto-hardened-aot lane, adds AssemblerPolicy::signsReturnAddresses (false for images), gates the PAC hints in MacroAssemblerARM64 on it, converts 28 removeCodePtrTag() calls on saved return addresses to removeReturnAddressSignature(), and makes frame-less LLInt thunks return without authenticating. It touches control-flow integrity (return-address signing) and stack walking/unwinding, and the author states no hardware with pointer authentication has run it. The remaining inline finding is a nit, but the security-sensitive surface, the untested-on-PAC status, and the one cosmetic image-identity deviation are why a human look is still worthwhile rather than approval.
It emitted XPACLRI and four moves whatever the policy. Its one user, emitRestoreInstanceFrameIfNeeded(), strips
the return address that the function's own prologue saved, which has no signature in an image. It did no harm
there, but an image with a cross-instance tail call of WebAssembly was not the same from a compiler with the
switch and from one without.
All seven places in MacroAssemblerARM64.h that emit a hint ask the policy now, and nothing else emits one.
testAssemblerPolicy counts this one too: six for the JIT, none for an image.
…ler, signed and not
linux-aarch64-hardened-aot-signed links the preview build of oven-sh/WebKit#968 at 3e9943227b, whose
bun-webkit-linux-arm64-lto-hardened-aot signs return addresses, and compiles Bun's own C, C++ and Rust with pac-ret.
linux-aarch64-hardened-aot-unsigned is the same without, to compare it with: such a lane has not run before.
Both run every test, test/aot included.
…e it returns
It came with main. It is not code of an image, so its ret() authenticates, and what it pops is what a stub of an image saved. With pointer authentication stubs/frames-of-slow-paths-have-no-word-that-nobody-wrote ended with SIGILL.
- Where the word comes from rawReturnPC(), returnPCForInspection() is the two calls together.
- EvacuatedStackSlice::implant() asserts that the frame it returns from holds a signed return address.
- emitRestoreInstanceFrameIfNeeded() asserts that its policy signs. No function with a tail call goes into an image, so removePtrTag() was never assembled with the policy of one: what 3e99432 says about such an image is wrong.
- Air::generate() asserts the policy in every build.
- testAssemblerPolicy calls all eight emitters.
- Two calls in testair go back to generate(), where the wrapper did nothing.
- lto-hardened-aot-no-webassembly has signedReturnAddresses like its twin. It is built for x64 alone, so no lane's flags change.
- Comments.
…is not on a list
On a saved return address it compiles, and fails only on a processor with pointer authentication, which not every runner of the test job has. The one on the list has an address that compiled code passes as an argument.
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Follows #945, which left the lanes with the ahead-of-time compiler out:
ENABLE(RETURN_ADDRESS_SIGNING)withENABLE(AOT)was an#error. This takes the#erroraway and givesbun-webkit-linux-arm64-lto-hardened-aotthe flags that-lto-hardenedhas. No other lane changes.What signs and what does not
-mbranch-protection=pac-retAn image does not sign, so that it is the same whether or not the compiler that makes it has the switch, and runs with a
bunthat is compiled this way and with one that is not. The instructions are hints, so code that has them signs on any processor with pointer authentication, whatever run time it is loaded by. And the run time does more with the return address of a compiled frame than read it: it writes one (setReturnPC()inAOTUnwinding.cpp) and compares it with the address of a stub as it is (AOTCoroutineTable.cpp,AOTJobs.cpp). All of that stays as it is.A frame without a signature among frames with one is nothing new: that is what a frame of a library compiled without the flag is. Each function checks only what it signed itself, and whatever walks the stack takes off a signature that may be there.
How
AssemblerPolicy::signsReturnAddresses, false inforImage(). Whatever goes into an image is assembled with that policy already, for its relocations.MacroAssemblerARM64asks it in the eight functions that emit a signing instruction (tagReturnAddress(),untagReturnAddress(), both forms oftagPtr()anduntagPtr(),removePtrTag(),validateUntaggedPtr(),ret()). None of the places inaot/that emit a prologue or a return changes.The run time takes the signature off what it reads. It reads the return address in the frame record of its own operations, which are C++ and sign, and those of whatever else is on the stack. The calls of
removeCodePtrTag()with a saved return address for an argument areremoveReturnAddressSignature()now, which isremoveCodePtrTag()everywhere else: 33 inaot/runtime, six of which came with #942, and two inStackVisitor.cpp. Ten of the 33 had the word fromrawReturnPC()and arereturnPCForInspection(), which is the two together. The one that is left inaot/has an address that compiled code passes as an argument.One of the two in
StackVisitor.cppisisCalledByCompiledCode(), new in #942, which compares the word with the bounds of the image's code. With a signature in it the address is in no image, and the visitor took compiled frames for the interpreter's.JSPI leaves the return addresses of an image's frames as they are.
EvacuatedStackSlicesigns the return address of every frame anew when it takes the frames off the stack and when it puts them back. What WebAssembly code of an image has saved has no signature to authenticate. The callee of the frame says whether its code is in an image. The word itself does not: a signature may be all zeros. The frame thatimplant()returns from is signed whatever its callee. WithSlicingStrategy::Slabit is the frame of the function that suspends, which is no code of an image, and aRELEASE_ASSERTsays so now.What lets Air emit the frame does not sign. With
frame.emitsFramea path that needs no frame sets none up, so it has signed nothing when it gets toret(). Only the ahead-of-time compiler sets that, with the policy of an image, andAir::generate()asserts it now.Ten returns in
LowLevelInterpreterAOT.asmare of thunks that make no frame (_llint_aot_char_code_at_thunkand its like), so they have not signed anything. offlineasm'sretauthenticates. They return withreturnWithoutFrame().In the tests: the two landings of
StubRunnerandCalleeThatReadsItsClosureend inret()without a frame and are not code of an image, so they begin withtagReturnAddress(). A third landing came withmainin the last merge. It pops what a stub of an image saved, so it signs behind the epilogue.testairsetsemitsFramewith the policy of the JIT, so its calls ofgenerate()go through a function that takes the signing off with it.AOTOperationScopeTests.cppandAOTSavedFrameTests.cpphad five moreremoveCodePtrTag().Also here: a comment of #945 in
wtf/PtrTag.hsaid that distributions compile the C library with the flag. I have no evidence for that, so it no longer says it.How I tested it
On hardware: at b2c96a0 and at 3e99432 the
testjobs got arm64 runners that have pointer authentication, and print so. Not all of that pool do: earlier the same day the jobs of #965 got runners without, and warned about it. InTest bun-webkit-linux-arm64-lto-hardened-aot, the lane of this PR, both times:testaot.irtests and the 2986 native tests that the program hastestmasm,testair,testb3run-javascriptcore-testsSo the 13 native tests that I cannot run, below, pass there with pointer authentication, and so do
webassembly-instance/suspends-and-goes-onand the one that is too slow for the emulator. All 85 checks pass at both commits. 16de3d0, the head, hasmainat f25de97 in it and what the second review found, below.In Bun's CI, on machines that print
paca: oven-sh/bun#47554 (a draft, not for merging) built Bun for Linux arm64 twice at the same commit, with the compiler in and hardened. One links the-lto-hardened-aotlane of 3e99432 and is compiled with-mbranch-protection=pac-retitself (C, C++ and Rust). The other, the control, signs nothing and links the same lane of #965 at 122e5da, which ismainat 4d45755, like 3e99432, with changes that are off in that lane. Build 136636:PACIASP, 156200AUTIASP, 70PACIBSP, 73AUTIBSPPACIBSPbun test --aotandbun run --aot. I have not looked at each of the others, only that the two lists are the same.test/aot/engine/native-1to-4, which aretestaot nativelinked with Bun's table, so with the tests that areLEFT OUThere, pass.native-6andir-1,-2,-4and-6fail in both.Under an emulator: everything below is
qemu-aarch64, with-cpu neoverse-n2, which has pointer authentication and faults where it fails, and-cpu neoverse-n1, which has none. The builds have the lane's flags but for LTO.What it was like with nothing but the
#errortaken away: of the 153.irtests that run a compiled program in the shell, 152 passed without pointer authentication and 3 with it.At 16de3d0, the head, which has
mainat f25de97 in it:.irtest, one run oftestaot runeachtestaot nativetestmasmtestairOK!testb3OK!The native tests that fail. One with pointer authentication alone:
valueSetsOfBothFormsAgreeWithPlainSets"printed nothing for 100 seconds". Alone and with--single-processit passes in 144 seconds, against 80 without the switch: the emulator computes every signature in software.16 fail on both. Two of them do now and then wherever I run them, and pass alone:
collector/helpers/small-heap-starts-noneandstartup/main-thread-stack-ends-above-its-first-frame.child-process/trap-is-reportedis new on that list. It has been in the tree since #942 and passed on the machine that I had then, and what it looks at is how the trap of a child is reported, which the emulator does. The other 13 say nothing about this PR, because they fail in my build whatever the processor, and without the switch too:stubs/call-method-by-id/closures-not-createdand 12 ofstubs/get-by-id,stubs/put-by-idandstubs/get-by-val. They put fake objects on the stack and depend on where those are, which is different under the emulator. They pass in thetestjob, above. Before that run I had only read: of the 20 places inaot/teststhat declare an assembler, the three named above were the ones that return without a frame, a signature or the policy of an image.What failed with pointer authentication alone on the way here, and passes now:
stubs/*withSIGILL, where a landing ofStubRunnerauthenticated what nothing had signed, andoperation-scope/the-frame-record-holds-the-return-address..irtests that throw or take a stack trace:isCalledByCompiledCode(), above.webassembly-instance/suspends-and-goes-on, which is new here and suspends frames of an image's code: without the change toEvacuatedStack.cppthe shell ends withSIGILL.stubs/frames-of-slow-paths-have-no-word-that-nobody-wrote, which came with it: its landing, above. Without thetagReturnAddress()it ends withSIGILLwith pointer authentication and passes without.The rest of this section is of 35a4d79, before the merges.
An image is the same. 26 images compiled by this build and by one without the switch are the same byte for byte. All eight places in
MacroAssemblerARM64.hthat emit a hint ask the policy.removePtrTag()has since 3e99432, whose message says that an image with a cross-instance tail call of WebAssembly differed until then. That was wrong: no function with a tail call goes into an image (UsesTailCallsinWasmOMGIRGenerator.cpp), so nothing assembled it with that policy.emitRestoreInstanceFrameIfNeeded(), its one caller, asserts now that its policy signs. One of them had 328PACIBSPand 1895AUTIBSPwith nothing but the#errortaken away, and has none now, with 1895retin both.Ordinary JavaScript, because the thunks without a frame are entered by any code:
JSTests/stresswith this build'sjsc, with pointer authentication.--useJIT=0The 1 is
bun-return-address-with-wrong-signature-is-not-returned-to.js, which must crash. My runner takes the options of a test's firstrunDefaultorrequireOptionsline, gives it 90 seconds and looks at nothing but the exit code, so tests fail in it for want of their options or of time. 19 other tests ended with a signal in one row or another, 16 of which set--watchdog. I ran each again on this build and on one without the switch, 10 times or more. 18 end so on both, at like rates: the 15 of the default row exactly as often on the one as on the other.The one that did not is
early-return-from-builtin.js: 18 of 60 runs here, 1 of 60 without the switch, 0 of 60 here without pointer authentication. It is the time that it takes, not the signature. It runs for 0.9 seconds here and for 0.7 in the two others, because the emulator computes signatures in software, and its watchdog is set to 1 second. With other settings, of 30 runs each:So a watchdog that fires in the middle of this test ends it with a segmentation fault under the emulator whatever the build. I have not looked at why. These tests pass in the
testjob, on arm64 hardware.Tools/Scripts/check-signed-return-addresses.pyon thejscof 16de3d0: 58440 functions sign, 19 are on the list.What keeps it true
testAssemblerPolicyintestmasm, which this lane builds and runs: all eight emitters andret()assembled withforImage()are one instruction, theret, and withforJIT()they have eight hints. It looks at the bytes, so it needs no pointer authentication.RELEASE_ASSERTinAir::generate(): what lets Air emit the frame is assembled with a policy that does not sign. It needs no pointer authentication either.webassembly-instance/suspends-and-goes-on, which tells only on a processor that has it, like every test that throws or takes a stack trace out of compiled code.Tools/Scripts/check-signed-return-addresses.py, behind the link in the lane.aot/: the same script reads the sources too, and fails whereaot/has aremoveCodePtrTag(that is not on its list, which has the one inAOTInstance.cpp. It needs no pointer authentication. With one planted it fails and names the file.Why the rule: a new
removeCodePtrTag()on a saved return address compiles, and #942 brought eleven in a day. Whether thetestjob sees one depends on which runner it gets. What still does not keep it true: a saved return address that is read with neither function, and code of a test that returns without a frame, like the landing above. Those show only on a runner with pointer authentication.What the review found
Three things that I had missed, all fixed above: JSPI,
testair, andCalleeThatReadsItsClosure, which is in one of the 13 tests that I cannot run. And thattestAssemblerPolicywould have failed for ARM64E, which signs withPACIBSPwhatever the policy says. It is not compiled for ARM64E now. No lane is. AndremovePtrTag(), above.A second review, of 3e99432, found no bug that can be reached. From it:
returnPCForInspection(), the two new assertions, the source rule,testAssemblerPolicycalling every emitter,signedReturnAddressesonlto-hardened-aot-no-webassembly(built for x64 alone, so no lane's flags change), two calls intestairback togenerate()where the wrapper did nothing, and the wrong sentence about tail calls.Left as it is: in this lane the four trampolines that sign for a function of the interpreter go on to the ordinary prologues, so
aotFunctionPrologue, which is faster, is not reached where an image runs. It costs speed and nothing else. Four of the strips inaot/runtimeare of a word that its caller has stripped already. Both hardened lanes for arm64 have thesignedReturnAddresseskey oflanes.mjsnow, andhardened-aot, which has not, is built for x64 alone. So the key could go, once #965 has landed too.With #965
#965 gives
-lto-hardenedlanding pads. The two do not touch the same lines oflanes.mjs. Landing pads are a key of their own there and an#errorwithENABLE(AOT), so this lane does not get them along with the signing, whichever lands first.Generated by Claude Code