Carry a fix for the async page fault / NMI race in every x86 guest kernel - #1020
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A restored guest panics with "Kernel panic - not syncing: Host injected
async #PF in interrupt disabled region" when perf samples with user call
chains. A restored clone takes a KVM async page fault for every page the
host has not paged in yet. If a sampling NMI arrives after one is
delivered and before its handler reads the per-CPU reason flags, and the
NMI's user stack walk faults, the nested fault reads and clears those
flags, sees interrupts disabled and panics. In a www dev VM it killed the
clone within about 20 s of starting `perf record -g`, 7 of 7 times, and
the guest's reboot fallback shows up in Firecracker's log as
"Unexpected exit reason on vcpu run: Shutdown".
Linux described this race in 2020 and did not fix it; 6.18.50 and
mainline have the same code. kernel/patches-default-x86/0002 reads the
flags only for a fault from user mode: the guest never enables
KVM_ASYNC_PF_SEND_ALWAYS, so the host delivers async page faults only to
user mode. A kernel-mode fault inside an NMI, #MC or #DB returns without
touching the flags, which stay for the fault they belong to. A
kernel-mode fault outside those handlers only checks whether a reason is
pending, and keeps the "async #PF in kernel mode" panic for a host that
delivers one.
The patch is x86-only, so it lives in x86-only patch directories and the
arm64 kernels and their kernel_sha do not change:
- the x86 default profile applies kernel/patches-default-x86, which also
links the virtio_ring fix from kernel/patches-default; its kernel_sha
changes to ce5378155114;
- the x86 btrfs profile applies kernel/patches-btrfs-x86, which links
kernel/patches plus this patch;
- the nested x86 profile links it from kernel/patches-x86 as a .vm.patch,
so the x86 host kernel does not apply it.
test_async_pf_inside_nmi_does_not_panic_a_restored_guest fills 1024 MiB in
a 4-vCPU, 2 GiB guest, snapshots it, restores it through a copy-mode serve
with prefetch off, and reads the memory back while every thread samples
hardware cycles with call chains and aims its frame pointer at an unmapped
page. It requires every thread's sampling event to open and the memory
server to report at least 10,000 demand faults.
only_the_x86_guest_kernels_apply_the_async_pf_nmi_fix fails when an x86
guest kernel stops applying the patch or an arm64 one starts.
Tested:
make _test-root FILTER=test_async_pf_inside_nmi (unpatched kernel 3b83b5face2d)
Error: the restored guest died while reading under call-chain sampling:
... [ 26.895685] Kernel panic - not syncing: Host injected async #PF in interrupt disabled region
TRY 2 FAIL, same panic at 25.9 s; Summary 1 test run: 0 passed, 1 failed
make _test-root FILTER=test_async_pf_inside_nmi (patched kernel ce5378155114)
clone survived 20 s of async page faults under call-chain sampling
the clone took 171517 demand faults
PASS [ 49.058s]; Summary 1 test run: 1 passed
make test-unit FILTER="-E 'binary(test_default_kernel_release)'"
34 tests run: 34 passed
with the patch linked into kernel/patches-arm64: FAILED,
the async #PF / NMI fix must be applied by exactly the x86_64 guest kernels: ["nested.arm64 applies=true"]
make lint: exit 0
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Included review availability: This review used your included allowance. Your plan provides up to 1 included review per hour; 0 remain after this review. 📝 WalkthroughWalkthroughThe amd64 default and btrfs kernel profiles now select x86-specific patch sets that include an async page-fault/NMI fix. The change adds a reproducer and tests patch selection across architectures and snapshot-clone execution. Arm64 profile selection remains unchanged. Changesx86 async page-fault fix
Priority: ⬆️ High Estimated code review effort: 3 (Moderate) | ~25 minutes Change: Bug fix Merge Risk: ⚪ Minimal · up to No outstanding issue identified in the x86 async page-fault fix; it is mergeable after normal checks. Security Architecture ReviewSecurity architecture risk: 🔵 Low · up to The change is intended to prevent restored x86 guests from crashing during page faults and does not add a production entrypoint or a demonstrated security exposure. Risk remains in rolling out a kernel-level change across all x86 guest profiles; the supplied evidence does not establish the behavior of published kernel binaries. Retained concerns Security review detailsSecurity Blast Radius
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- 🪄 Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @tests/test_snapshot_clone.rs:
- Line 4631: Update the patch reference in the doc comment near the snapshot
clone test to use the full x86 default-profile patch path and filename,
replacing the incorrect `kernel/patches-default/0002` reference.
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kernel/README.mdkernel/default-amd64.build.tomlkernel/patches-btrfs-x86/0001-fuse-add-remap_file_range-support.patchkernel/patches-btrfs-x86/0002-fuse-fix-utimensat-with-default-permissions.patchkernel/patches-btrfs-x86/0003-virtio_ring-fix-infinite-loop-in-virtnet_poll_cleantx.patchkernel/patches-btrfs-x86/0004-x86-kvm-leave-the-async-pf-reason-for-the-fault-it-belongs-to.patchkernel/patches-default-x86/0001-virtio_ring-fix-infinite-loop-in-virtnet_poll_cleantx.patchkernel/patches-default-x86/0002-x86-kvm-leave-the-async-pf-reason-for-the-fault-it-belongs-to.patchkernel/patches-x86/0004-x86-kvm-leave-the-async-pf-reason-for-the-fault-it-belongs-to.vm.patchrootfs-config.tomltests/data/async_pf_nmi.ctests/test_default_kernel_release.rstests/test_snapshot_clone.rs
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The Firecracker fork zeroes CPUID leaf 0xA for Intel guests (update_performance_monitoring_entry), so an Intel guest has no hardware events and perf cannot raise the NMIs the race needs. On the c5.metal CI runners the reproducer reported "perf_event_open: No such file or directory" and "sampling events opened: 0 of 4", and the test failed. On an Intel host the test now asserts exactly that, so a Firecracker change that starts exposing the PMU there turns it red. AMD hosts keep the full check. Also names the patch by its full path in the doc comment. Tested: make test-root FILTER="-E 'test(=test_async_pf_inside_nmi_does_not_panic_a_restored_guest)'" on an AMD EPYC Genoa host: clone survived 20 s with 4 of 4 sampling events, 171806 demand faults, 1 passed.
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NOT-A-DEFECT: CodeRabbit's review bodies carry one finding, the doc-comment patch path, answered inline and fixed in 73daa6a. 73daa6a also fixes the Host-Root-x64-SnapshotEnabled failure: Intel Firecracker guests have no PMU (leaf 0xA zeroed), so the test now asserts that on Intel and keeps the full check on AMD.
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A restored guest panics with
Kernel panic - not syncing: Host injected async #PF in interrupt disabled regionwhen perf samples with user call chains. This carries a guest kernel patch that fixes it, for every x86 guest kernel.The problem
A restored clone takes a KVM async page fault for every page the host has not paged in yet. If a perf sampling NMI arrives after one is delivered and before its handler reads the per-CPU reason flags, and the NMI's user stack walk faults, the nested fault reads and clears the outer fault's flags, sees interrupts disabled, and panics. The guest then reboots through its triple-fault fallback, which Firecracker logs as
Unexpected exit reason on vcpu run: Shutdown. In a www dev VM,perf record -gkilled the clone within about 20 s, 7 of 7 times. Linux described this race in 2020 and left it open; 6.18.50 and mainline have the same code.The fix
kernel/patches-default-x86/0002reads the flags only for a fault from user mode, the only mode the host delivers async page faults to (the guest never setsKVM_ASYNC_PF_SEND_ALWAYS). A kernel-mode fault inside an NMI, #MC or #DB returns without touching the flags. A kernel-mode fault outside those handlers keeps the existing "async #PF in kernel mode" panic.The patch lives in x86-only directories, so the arm64 kernels and their
kernel_shado not change. The x86 default kernel'skernel_shachanges toce5378155114, so it needs a new release. The x86 btrfs kernel getskernel/patches-btrfs-x86, and the nested x86 kernel links the patch as a.vm.patchso the host kernel does not apply it.Contract and evidence
Contract: every x86 guest kernel applies the patch, no arm64 kernel does, and a restored guest survives call-chain sampling while taking async page faults.
The VM test is x86-only. Its reproducer (
tests/data/async_pf_nmi.c) is built with the host'scc. The panic needs a guest PMU, and only AMD hosts give a Firecracker guest one: the Firecracker fork zeroes CPUID leaf 0xA for Intel guests (update_performance_monitoring_entry). So on an AMD host the test requires every thread's sampling event to open and the clone to survive, and on an Intel host (the c5.metal CI runners) it asserts thatperf_event_openfails with ENOENT on every thread, which turns red if Firecracker ever exposes the PMU there. The red and green runs above are from an AMD EPYC Genoa host; CI's x64 jobs exercise only the Intel branch.Summary by CodeRabbit