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[JSC] Write-protect the JIT region with a memory protection key on Linux x86-64 - #970
On Linux x86-64 the JIT region is a single 1 GiB mapping that stays rwx for the life of the process. Any bug that gives an arbitrary write therefore gives code execution: write shellcode over a hot function and wait for it to be called. macOS arm64 has had per-thread JIT write protection for years. Linux x64 has nothing.
Fix
Where the CPU and the kernel support memory protection keys (PKU: Intel server parts since Skylake-SP, client since Tiger Lake, AMD since Zen 3), give the region a key at startup, and implement WebKit's existing per-thread abstraction with it.
threadSelfRestrict<kRwxToRw / kRwxToRx>() gets a Linux x86-64 implementation in FastJITPermissions.h, and g_jscConfig.useFastJITPermissions is on where the region has a key. performJITMemcpy() is unchanged: it already opens the region for the copying thread and closes it again.
X86Assembler's patching functions (setInt32, setPointer, replaceWithJump, ...) go through performJITMemcpy(). Upstream already has those paths for ENABLE(MPROTECT_RX_TO_RWX), so this only widens their #if.
Every thread can execute and read the region. A thread can write to it only between those two calls.
useJITProtectionKey=0 (BUN_JSC_useJITProtectionKey=0) turns it off. --verboseExecutablePoolAllocation=1 says whether the region got a key.
V8 has the same design (V8_HAS_PKU_JIT_WRITE_PROTECT).
Known regression: signal handlers that read JIT code
The kernel starts every signal handler with no access to memory that has a protection key. A handler can still run while JIT code is interrupted, but if it reads JIT bytes as data it faults. This is inherent to protection keys. V8 has the same limitation and gives embedders a function to call at the top of their handlers; here that is JSC::setJITRegionWritableForCurrentThread(false).
glibc's backtrace() does such a read: for a pc with no unwind information, libgcc looks at the bytes there. Reproduced in Bun with a native library whose SIGPROF handler calls backtrace() while a hot loop runs:
Result
key off
finishes, 106 samples
key on
Segmentation fault at an address in the JIT region
Bun's own crash handler is not affected. Its reports for SIGSEGV, SIGILL, SIGTRAP, SIGFPE and SIGBUS arriving in JIT code are the same with the key on and off.
This is the main argument against having the option on by default, and it is an open question for this PR.
When the region gets no key
In each of these cases behaviour is exactly as before:
Condition
Why
CPUID does not report OSPKE
No hardware or kernel support.
Linux < 5.13, except 5.4.182+ and 5.10.103+
Those kernels can lose PKRU across a context switch ("x86/fpu: Correct pkru/xstate inconsistency"). V8 has the same check.
The thread is under a seccomp filter
See below. useJITProtectionKeyUnderSeccompFilter=1 overrides this.
All 15 keys are taken
seccomp. Nothing tells a process what a filter would do with a call, and a filter may kill it. Docker's default profile allows these calls. systemd's @system-service does not list them, and systemd kills by default. I traced a Bun service (HTTP server, fetch, files, SQLite, worker, subprocess, crypto): all 68 system calls it makes are in @system-service, so Bun works under that filter today, and a pkey_alloc at startup would kill it with SIGSYS.
So under a filter the region gets no key, and startup makes no kind of system call that it did not make before. Whether there is a filter comes from the Seccomp: line of /proc/thread-self/status, not from prctl(PR_GET_SECCOMP): a filter can single that call out by its argument, but it cannot tell one path from another. Where the file cannot be read, that counts as a filter.
The cost: no key by default in Docker and other containers that install a filter.
Threads
A new thread inherits its creator's rights. A thread older than the key cannot even read the region, so JSLock::didAcquireLock() gives it read access. Any write also leaves the thread with read access.
What this does and does not stop
It stops an attacker who has an arbitrary write from writing into JIT memory. It does not stop one who already controls the instruction pointer and can call the function that opens the region. It is a mitigation that removes the easiest step of an exploit, not a sandbox.
Cost, against unmodified WebKit
jsc built from this branch and from its merge base, interleaved runs, medians:
Workload
No key (only the performJITMemcpy() routing)
With the key
6,000 distinct functions, each compiled by several tiers (worst case on purpose)
wall -0.12%, CPU +0.17%
wall +0.68%, CPU +1.22%
SunSpider, all 26 in one run
wall +0.83%, CPU +0.32%
wall +0.37%, CPU +0.17%
Everything but the worst case with the key is noise. .text grows by 29 KB (0.09%). One patch with the key costs about 25 ns, against about 4,000 ns for an mprotect pair.
Verification
On an AMD Zen 4 VM with PKU, Linux 6.18.
It works. With bun:ffi as a stand-in for an arbitrary write into the region, measured on an earlier revision with the same key setup: Bun 1.4.4 and this branch with the option off let the write through and keep running; this branch stops it with Segmentation fault.
Under real seccomp filters (testmasm, current revision), including an allow-list rebuilt from systemd's source for @system-service:
Filter
Default
With useJITProtectionKeyUnderSeccompFilter=1
none
starts, key on
allows everything (Docker-like)
starts, no key
starts, key on
kills for pkey_*
starts, no key
SIGSYS, as asked for
@system-service
starts, no key
SIGSYS, as asked for
kills process creation
starts, no key
kills prctl(PR_GET_SECCOMP)
starts, no key
Current revision:testmasm (448) passes with the key on and off. Signal-based VM traps (--watchdog --usePollingTraps=false) stop a hot loop. check-webkit-style reports 0 errors. Disassembly shows RDPKRU and WRPKRU in one function only.
Earlier revisions, same write paths in X86Assembler, a different copy function:
run-javascriptcore-tests as CI invokes it, with the key on: 126,593 runs, no crashes. The 20 runs that failed locally also failed with the option off, and do not fail in CI.
Debug + ASan jsc, a subset around ICs, jettison, watchpoints, OSR, termination and Wasm: 24,322 runs, no assertion failures, ASan reports or crashes.
Bun's CI (DO NOT MERGE: test WebKit preview build with the JIT protection key bun#47595) on Intel Sapphire Rapids. Debian 13, Debian 13 asan and Ubuntu 25.04 x64 have protection keys, and a test there confirms that the JIT region had its key. No failure came from the protection itself. Alpine 3.23 x64 reports pku without ospke, so the region has no key there.
The full suites have not been re-run on the current revision. Bun's CI will do that once oven-sh/bun#47595 moves to this revision's preview build.
New tests in testmasm.
testJITProtectionKey checks what a thread may do to the region (through a pipe, so nothing faults), inheritance by new threads, patching and reading in a signal handler, a thread with no rights, and the JSLock call site.
testJITProtectionKeyUnderSeccompFilter re-executes testmasm under a filter that kills for pkey_alloc and for prctl(PR_GET_SECCOMP). It must start normally by default, and die with SIGSYS with the option on.
Removing the JSLock grant, leaving the region writable after a copy, and asking for a key under a filter each make them fail.
Limitations
Signal handlers that read JIT code, above.
A thread that leaves a signal handler with siglongjmp keeps the handler's rights (no read access) until it next patches code or takes a VM's lock. Bun never does this; a native addon could.
No key by default under a seccomp filter, which includes Docker, or where /proc is not mounted.
Each patch opens and closes the region by itself. Nothing batches the link tasks of a LinkBuffer.
Linux x86-64 only. Android is compiled out.
CI
linux-x64-gh has no PKU, so green CI here covers the path without a key and not the protection itself. The test job now asks jsc whether its JIT region has a key and warns when it does not. A later change that writes to the JIT region directly would pass this repository's CI, and be caught in Bun's CI at the next WebKit bump.
Follow-up in Bun when WebKit is bumped
RDPKRU and WRPKRU are newer than the Nehalem baseline. They are in one function, which scripts/verify-baseline-static/allowlist-x64.txt needs, with feature PKU:
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Inline comments:
Review comments at @Source/JavaScriptCore/assembler/X86Assembler.h:
- Line 6837: Update the protected-copy encoding used by revertJumpTo_movl_i32r
to use the same REX byte as the direct-write branch, removing REX.W so the
32-bit move consumes only its intended immediate.
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Walkthrough
The changes add Linux x86-64 JIT protection-key configuration and setup, thread permission handling, protected assembler patching paths, tests, and a CI check for protection-key reporting.
Changes
JIT Protection-Key Support
Layer / File(s)
Summary
Feature configuration and key setup Source/WTF/wtf/PlatformEnable.h, Source/JavaScriptCore/runtime/OptionsList.h, Source/JavaScriptCore/runtime/JSCConfig.h, Source/JavaScriptCore/jit/ExecutableAllocator.cpp
The feature defaults and runtime options are added, and JSC::Config stores the key bits. The allocator checks platform, kernel, and seccomp conditions before allocating and applying a key to the JIT region.
Keyed writes and assembler repatching Source/JavaScriptCore/assembler/X86Assembler.h
Assembler replacement, jump reversion, NOP filling, and pointer or integer patching use protected-copy paths when JIT protection keys or MPROTECT_RX_TO_RWX are enabled. The direct-write path in revertJumpTo_movl_i32r changes its REX prefix.
FastJITPermissions handles the supported JIT permission restrictions. When fast JIT permissions are enabled, JSLock::didAcquireLock() restricts the current thread’s JIT-region permissions from RWX to RX.
Protection-key tests and CI detection Source/JavaScriptCore/assembler/testmasm.cpp, .github/workflows/ci.yml
Tests cover JIT execution, reads, writes, and repatching across threads and a signal handler, plus restricted access and seccomp startup behavior. Linux CI checks verbose executable-pool output for protection-key reporting on x86_64.
Suggested reviewers: jonwbedard, ddegazio
Merge Risk: Moderate · up to 26488
Reverting a patchable JIT branch can execute an incorrect instruction stream on systems using a protection key. Fix the encoding before merging.
Pre-merge checks: 4
Check name
Status
Explanation
Title check
The title clearly and concisely describes the main change: adding memory-protection-key write protection for the JIT region on Linux x86-64.
Description check
The description is directly related to the changeset. It explains the protection-key implementation, affected code paths, options, limitations, tests, and validation results.
Linked Issues check
Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check
Check skipped because no linked issues were found for this pull request.
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Actionable comments posted: 2
Fix comments: Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @Source/JavaScriptCore/jit/ExecutableAllocator.cpp:
- Around line 622-628: Update the kernel-release parsing in the visible
version-check function to accept releases with only major and minor components.
Initialize patch to zero and reject the parse only when fewer than two
components are found, preserving the existing version checks for releases with
or without a patch number.
Review comments at @Source/WTF/wtf/PlatformEnable.h:
- Around line 1050-1052: Update the ENABLE_JIT_PROTECTION_KEY default condition
in PlatformEnable.h to also require USE(BUN_JSC_ADDITIONS), preserving its
existing platform and JIT checks so the default is enabled only for Bun JSC
additions builds.
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Caution
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⚠️ Outside diff range comments (1)
Minor · Handle signal termination as a failed seccomp probe. · testmasm.cpp:11453-11477
Source/JavaScriptCore/assembler/testmasm.cpp:11453-11477 Minor · Stability & Availability · Quick win
Handle signal termination as a failed seccomp probe.
The kill and trap filters terminate the child by signal. WIFEXITED(status) is therefore false, and the unconditional CHECK_EQ aborts testmasm before the probe returns false. Handle WIFSIGNALED(status) as the expected failed probe while retaining the exit-status handling for normally exiting children.
Suggested fix
int status;
RELEASE_ASSERT(waitpid(child, &status, 0) == child);
+ if (WIFSIGNALED(status))+ return false;
CHECK_EQ(WIFEXITED(status), true);
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Review comment at @Source/JavaScriptCore/assembler/testmasm.cpp around lines
11453 - 11477:
Update protectionKeySystemCallsReturnUnderFilter to treat a child terminated by
a signal as a failed probe by returning false when WIFSIGNALED(status) is true.
Preserve the existing exit-status handling for children that exit normally.
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Outside diff comments:
Review comments at @Source/JavaScriptCore/assembler/testmasm.cpp:
- Around line 11453-11477: Update protectionKeySystemCallsReturnUnderFilter to
treat a child terminated by a signal as a failed probe by returning false when
WIFSIGNALED(status) is true. Preserve the existing exit-status handling for
children that exit normally.
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On the CodeRabbit finding about protectionKeySystemCallsReturnUnderFilter in testmasm.cpp ("Handle signal termination as a failed seccomp probe"): leaving this as it is.
There are two levels of child here. The test forks a process and installs the filter in it, and that process calls protectionKeySystemCallsReturn(), which creates its own short-lived child to make the system calls. The filter kills that inner child. The process the test waits for has to survive and report the answer, and CHECK_EQ(WIFEXITED(status), true) is the assertion that it did.
Returning false on WIFSIGNALED would make the test pass when the probe is broken and its caller gets killed, which is the failure it exists to catch. The test passes as written under KILL_PROCESS, KILL_THREAD and TRAP.
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Beyond the inline finding, two other things were checked and ruled out: the REX byte change in revertJumpTo_movl_i32r (X86Assembler.h:6839) now matches the direct-store variant at line 6844, so the emitted encoding is unchanged; and the new early return in performJITMemcpy() (ExecutableAllocator.h:309) skips jitMemcpyCheckForZeros, but that check is only compiled on x86-64 macOS, so nothing is lost on the Linux lanes where this flag is on.
Extended reasoning...
The change adds PKU-based write protection for the JIT region on Linux x86-64: raw pkey syscalls, a seccomp probe via SYS_clone, rdpkru/wrpkru inline asm, and rerouting of X86Assembler patch sites through performJITMemcpy. It is security-sensitive memory-protection code with a confirmed startup-crash finding under restrictive seccomp filters, so it is not a candidate for approval and a human look is warranted.
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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/jit/ExecutableAllocator.cpp:
- Line 638: Replace the PR_GET_SECCOMP check with a detection path that can fail
without terminating the process, and treat an unreadable result as filtered.
Preserve the useJITProtectionKeyUnderSeccompFilter() opt-out so it can disable
key setup even when PR_GET_SECCOMP is blocked.
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Fix comments: Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @Source/JavaScriptCore/jit/ExecutableAllocator.cpp:
- Line 646: Update the Seccomp value parsing around the `strtol` call so it
treats the thread as unfiltered only when the value is complete, valid, and
zero; treat a missing value or one with no digits because the read ended at the
buffer boundary as filtered.
- Line 635: Change the `/proc/thread-self/status` probe at the `open` call so
seccomp detection does not invoke a syscall that may terminate the process after
filters are installed. Arrange for detection before filters are installed, or
require host-provided state and default to no key when that state is
unavailable.
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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 raw SYS_clone probe I flagged on the earlier version is gone from giveJITRegionAProtectionKey(), so that concern is addressed by the current code. Beyond the inline finding, I also checked the new /proc/thread-self/status heuristic in mayBeUnderSeccompFilter(): a status file longer than the 4 KiB buffer (e.g. a very long Groups: line), a missing Seccomp: line on a kernel without CONFIG_SECCOMP, or an unreadable /proc all fail closed to "no key" rather than issuing pkey_alloc, so none of them reintroduces the SIGSYS-at-startup path.
Extended reasoning...
The diff adds ENABLE(JIT_PROTECTION_KEY), a Bun-only Linux x86-64 feature in ExecutableAllocator.cpp/.h, X86Assembler.h, JSLock.cpp, VMTraps.cpp, JSCConfig.h, OptionsList.h, PlatformEnable.h and a testmasm test, which gives the JIT region a PKU key and routes every repatch through a WRPKRU-guarded copy. It is a security mitigation on the JIT write path with per-thread PKRU state inherited across thread creation and reset in signal handlers, so it is security-sensitive and not approvable on an automated read alone; inline findings also exist. The commits since the prior review replaced the clone-based seccomp probe with a /proc-based check, and that new check was examined this run and found to fail closed.
…nux x86-64
The JIT region on Linux x86-64 is one mapping that is readable, writable and
executable for the life of the process, so a bug that gives an arbitrary write
gives code execution.
Where the CPU and the kernel have memory protection keys (PKU), give the region
a key, and implement threadSelfRestrict() with it. g_jscConfig.useFastJITPermissions
is then on, and performJITMemcpy() opens the region for the thread that copies,
and closes it again, as it does on Darwin. X86Assembler's patching functions go
through performJITMemcpy(), on the paths that ENABLE(MPROTECT_RX_TO_RWX) has.
The region has no key, and everything is as before, where CPUID does not report
OSPKE, where the kernel can lose PKRU across a context switch (before 5.13, but
for 5.4.182+ and 5.10.103+), under a seccomp filter, which may kill the process
for a system call that it does not list, or with useJITProtectionKey=0.
useJITProtectionKeyUnderSeccompFilter=1 is for a filter that allows the calls.
A thread older than the key cannot even read the region. It gets to when it takes
the lock of a VM. Neither can a signal handler, until it calls
setJITRegionWritableForCurrentThread(false).
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Actionable comments posted: 1
Fix comments: Fix CodeRabbit comments on this PR
Prompt to fix review comments
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Inline comments:
Review comments at @Source/JavaScriptCore/assembler/X86Assembler.h:
- Line 6837: Update the protected-copy encoding used by revertJumpTo_movl_i32r
to use the same REX byte as the direct-write branch, removing REX.W so the
32-bit move consumes only its intended immediate.
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Major · Stability & Availability · Quick win
Remove REX.W from the selected 32-bit move encoding.
When JIT_PROTECTION_KEY enables this branch, revertJumpTo_movl_i32r writes a six-byte instruction with REX.W. That prefix makes MOV consume an eight-byte immediate, including four bytes after the copied instruction. Reverting a patchable 32-bit branch can therefore execute an incorrect instruction stream. Match the direct-write branch’s REX byte in the protected-copy branch. (raw.githubusercontent.com)
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Review comment at @Source/JavaScriptCore/assembler/X86Assembler.h at line 6837:
Update the protected-copy encoding used by revertJumpTo_movl_i32r to use the
same REX byte as the direct-write branch, removing REX.W so the 32-bit move
consumes only its intended immediate.
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The encoding is wrong as you say, but nothing can execute it: revertJumpTo_movl_i32r is dead code. Its only caller is revertJumpReplacementToPatchableBranch32WithPatch in MacroAssemblerX86_64.h, which has no callers. The byte is upstream's, in a branch that no configuration of this fork compiled before.
An earlier revision of this PR did fix the byte. I took that back to avoid diverging from upstream in code that never runs, which was the wrong half of the choice: it left the branch enabled.
The next push leaves these functions exactly as upstream has them, #if included:
revertJumpTo_movq_i64r, revertJumpTo_movl_i32r, revertJumpTo_cmpl_ir_force32, revertJumpTo_cmpl_im_force32: no callers outside three wrappers that are themselves unused.
setInt8: no callers.
Linux then compiles the direct-write variants, whose movl encoding is correct, and this PR changes five sites in the file instead of ten. If one of them ever gets a caller, its direct write to the JIT region faults at once on a machine with protection keys, which Bun's CI has. That is a loud failure rather than a wrong instruction.
It will go in with the next code push, which is waiting on a decision about the option's default.
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@dylan-conway Understood. My comment identified the encoding error, but overstated the runtime impact without establishing a reachable caller.
Given the unused call chain you describe, restoring the upstream implementations and their original #if guards is a reasonable way to limit this PR’s scope. There is no need to change the upstream encoding as part of that approach.
The guard changes are still pending, so this thread remains open pending the next push. The relevant checks are the restored guards and the absence of reachable callers, not a separate encoding fix.
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline nit, I checked that the points from my earlier reviews are addressed in this push: startup no longer issues a SYS_clone probe (mayBeUnderSeccompFilter reads /proc/thread-self/status instead, so no new system call is made under a filter), and the testmasm child now puts its JSC_useJITProtectionKeyUnderSeccompFilter entry ahead of the inherited environment.
I also checked the PKRU bit arithmetic in ExecutableAllocator.cpp and testmasm.cpp: jitProtectionKeyBits covers both bits of the key, setJITRegionWritableForCurrentThread toggles only the odd write-disable bit and always clears access-disable, and takeAwayRightsToJITRegion sets only the even access-disable bit — consistent with the inheritance model the JSLock call site relies on.
Extended reasoning...
The push replaces the earlier clone-based seccomp probe with a read of /proc/thread-self/status and fixes the environment ordering in the testmasm seccomp test, which closes both of my prior inline threads. The change still adds a new JIT write-protection mechanism (raw pkey syscalls, RDPKRU/WRPKRU, per-thread rights touched from JSLock and signal handlers) across the allocator, assembler repatch paths and testmasm, so a human look remains worthwhile alongside the posted inline nit.
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Problem
On Linux x86-64 the JIT region is a single 1 GiB mapping that stays
rwxfor the life of the process. Any bug that gives an arbitrary write therefore gives code execution: write shellcode over a hot function and wait for it to be called. macOS arm64 has had per-thread JIT write protection for years. Linux x64 has nothing.Fix
Where the CPU and the kernel support memory protection keys (PKU: Intel server parts since Skylake-SP, client since Tiger Lake, AMD since Zen 3), give the region a key at startup, and implement WebKit's existing per-thread abstraction with it.
threadSelfRestrict<kRwxToRw / kRwxToRx>()gets a Linux x86-64 implementation inFastJITPermissions.h, andg_jscConfig.useFastJITPermissionsis on where the region has a key.performJITMemcpy()is unchanged: it already opens the region for the copying thread and closes it again.setInt32,setPointer,replaceWithJump, ...) go throughperformJITMemcpy(). Upstream already has those paths forENABLE(MPROTECT_RX_TO_RWX), so this only widens their#if.useJITProtectionKey=0(BUN_JSC_useJITProtectionKey=0) turns it off.--verboseExecutablePoolAllocation=1says whether the region got a key.V8 has the same design (
V8_HAS_PKU_JIT_WRITE_PROTECT).Known regression: signal handlers that read JIT code
The kernel starts every signal handler with no access to memory that has a protection key. A handler can still run while JIT code is interrupted, but if it reads JIT bytes as data it faults. This is inherent to protection keys. V8 has the same limitation and gives embedders a function to call at the top of their handlers; here that is
JSC::setJITRegionWritableForCurrentThread(false).glibc's
backtrace()does such a read: for apcwith no unwind information, libgcc looks at the bytes there. Reproduced in Bun with a native library whoseSIGPROFhandler callsbacktrace()while a hot loop runs:Segmentation faultat an address in the JIT regionBun's own crash handler is not affected. Its reports for SIGSEGV, SIGILL, SIGTRAP, SIGFPE and SIGBUS arriving in JIT code are the same with the key on and off.
This is the main argument against having the option on by default, and it is an open question for this PR.
When the region gets no key
In each of these cases behaviour is exactly as before:
useJITProtectionKeyUnderSeccompFilter=1overrides this.seccomp. Nothing tells a process what a filter would do with a call, and a filter may kill it. Docker's default profile allows these calls. systemd's
@system-servicedoes not list them, and systemd kills by default. I traced a Bun service (HTTP server, fetch, files, SQLite, worker, subprocess, crypto): all 68 system calls it makes are in@system-service, so Bun works under that filter today, and apkey_allocat startup would kill it with SIGSYS.So under a filter the region gets no key, and startup makes no kind of system call that it did not make before. Whether there is a filter comes from the
Seccomp:line of/proc/thread-self/status, not fromprctl(PR_GET_SECCOMP): a filter can single that call out by its argument, but it cannot tell one path from another. Where the file cannot be read, that counts as a filter.The cost: no key by default in Docker and other containers that install a filter.
Threads
A new thread inherits its creator's rights. A thread older than the key cannot even read the region, so
JSLock::didAcquireLock()gives it read access. Any write also leaves the thread with read access.What this does and does not stop
It stops an attacker who has an arbitrary write from writing into JIT memory. It does not stop one who already controls the instruction pointer and can call the function that opens the region. It is a mitigation that removes the easiest step of an exploit, not a sandbox.
Cost, against unmodified WebKit
jscbuilt from this branch and from its merge base, interleaved runs, medians:performJITMemcpy()routing)Everything but the worst case with the key is noise.
.textgrows by 29 KB (0.09%). One patch with the key costs about 25 ns, against about 4,000 ns for anmprotectpair.Verification
On an AMD Zen 4 VM with PKU, Linux 6.18.
It works. With
bun:ffias a stand-in for an arbitrary write into the region, measured on an earlier revision with the same key setup: Bun 1.4.4 and this branch with the option off let the write through and keep running; this branch stops it withSegmentation fault.Under real seccomp filters (
testmasm, current revision), including an allow-list rebuilt from systemd's source for@system-service:useJITProtectionKeyUnderSeccompFilter=1pkey_*@system-serviceprctl(PR_GET_SECCOMP)Current revision:
testmasm(448) passes with the key on and off. Signal-based VM traps (--watchdog --usePollingTraps=false) stop a hot loop.check-webkit-stylereports 0 errors. Disassembly showsRDPKRUandWRPKRUin one function only.Earlier revisions, same write paths in X86Assembler, a different copy function:
run-javascriptcore-testsas CI invokes it, with the key on: 126,593 runs, no crashes. The 20 runs that failed locally also failed with the option off, and do not fail in CI.jsc, a subset around ICs, jettison, watchpoints, OSR, termination and Wasm: 24,322 runs, no assertion failures, ASan reports or crashes.pkuwithoutospke, so the region has no key there.The full suites have not been re-run on the current revision. Bun's CI will do that once oven-sh/bun#47595 moves to this revision's preview build.
New tests in testmasm.
testJITProtectionKeychecks what a thread may do to the region (through a pipe, so nothing faults), inheritance by new threads, patching and reading in a signal handler, a thread with no rights, and theJSLockcall site.testJITProtectionKeyUnderSeccompFilterre-executestestmasmunder a filter that kills forpkey_allocand forprctl(PR_GET_SECCOMP). It must start normally by default, and die with SIGSYS with the option on.JSLockgrant, leaving the region writable after a copy, and asking for a key under a filter each make them fail.Limitations
siglongjmpkeeps the handler's rights (no read access) until it next patches code or takes a VM's lock. Bun never does this; a native addon could./procis not mounted.LinkBuffer.CI
linux-x64-ghhas no PKU, so green CI here covers the path without a key and not the protection itself. The test job now asksjscwhether its JIT region has a key and warns when it does not. A later change that writes to the JIT region directly would pass this repository's CI, and be caught in Bun's CI at the next WebKit bump.Follow-up in Bun when WebKit is bumped
RDPKRUandWRPKRUare newer than the Nehalem baseline. They are in one function, whichscripts/verify-baseline-static/allowlist-x64.txtneeds, with featurePKU:Generated by Claude Code