CVE-2026-99178: x86/kvm: Don't consume async #PF flags on a kernel #PF in NMI context

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

x86/kvm: Don't consume async #PF flags on a kernel #PF in NMI context

An async #PF "page not present" event arrives through the #PF vector,
and its reason sits in the per-CPU apf_reason.flags until
__kvm_handle_async_pf() reads and clears it. If an NMI arrives after
the event is injected and before the #PF handler reads the flags, and
the NMI handler itself takes a #PF, that fault reads and clears the
flags belonging to the #PF it interrupted. It then finds interrupts
disabled in the NMI context it interrupted, and panics:

  Kernel panic - not syncing: Host injected async #PF in interrupt
disabled region
  Call Trace:
   <NMI>
   panic+0x52/0x60
   __kvm_handle_async_pf+0xb0/0xb0
   exc_page_fault+0x71/0xb0
   asm_exc_page_fault+0x27/0x30
  RIP: 0010:__get_user_nocheck_8+0x6/0x20
   perf_callchain_user+0x147/0x1a0
   get_perf_callchain+0x12d/0x1c0
   ...
   perf_event_nmi_handler+0x26/0x50
   exc_nmi+0xe8/0x110
  RIP: 0010:asm_exc_page_fault+0x0/0x30
   </NMI>

The last RIP line shows the NMI landing on the first instruction of
the #PF entry. perf with user call chains takes such a fault: the
sampling NMI walks the user stack with __get_user(), which faults when
a saved frame pointer is not a valid user address or the walk runs off
the top of the stack. The same window exists when KVM, running as an
L1 hypervisor with DELIVERY_AS_PF_VMEXIT, reads the flags itself after
a #PF VM-exit.

The NMI entry code saves and restores CR2 for this nesting, but not the
async #PF reason. Andy Lutomirski described the race, with perf as the
example, when async #PF delivery was reworked in 2020 [1]. Saving and
clearing the flags across the NMI, next to CR2, was discussed there but
not merged.

Since that rework, Linux has stopped setting KVM_ASYNC_PF_SEND_ALWAYS,
so the host delivers "page not present" only for user mode faults and
a kernel-mode fault in NMI context (including kernel-mode #MC and #DB)
cannot be an async page fault.  In that case, return without touching
the flags so that the interrupted handler finds them intact.

Tested on v7.3-rc5 in a 4-vCPU, 2 GiB guest restored from a snapshot
whose memory is served on demand through userfaultfd, so that reading
it back takes async #PFs, while every thread samples hardware cycles
with user call chains and points its frame pointer at an unmapped page.
Without this patch the guest panicked as above in 3 of 3 runs. With it,
the guest survived 3 of 3 runs of 60 seconds, and the host counted
148,441 to 153,334 kvm_async_pf_not_present events in the first 20
seconds of each. On 6.18.50 the results were the same: the guest
panicked without the patch and survived with it.

The Linux kernel CVE team has assigned CVE-2026-99178 to this issue.


Affected and fixed versions
===========================

	Fixed in 6.18.59 with commit b77e9622bbade9812e2090c66d25c372f515e921
	Fixed in 7.2.13 with commit f45ee1ff6358a22d7e22a4c53cb3148ea97b10d9
	Fixed in 7.3 with commit 81d99abe2bd81379abfb595e403bbbee6fbfc624

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-99178
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	arch/x86/kernel/kvm.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/b77e9622bbade9812e2090c66d25c372f515e921
	https://git.kernel.org/stable/c/f45ee1ff6358a22d7e22a4c53cb3148ea97b10d9
	https://git.kernel.org/stable/c/81d99abe2bd81379abfb595e403bbbee6fbfc624