linux-cve-announce - CVE-2025-40026: KVM: x86: Don't (re)check L1 intercepts when completing userspace I/O

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Message-ID: <2025102841-CVE-2025-40026-760b@gregkh>
Date: Tue, 28 Oct 2025 10:32:41 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2025-40026: KVM: x86: Don't (re)check L1 intercepts when completing userspace I/O

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

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

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

KVM: x86: Don't (re)check L1 intercepts when completing userspace I/O

When completing emulation of instruction that generated a userspace exit
for I/O, don't recheck L1 intercepts as KVM has already finished that
phase of instruction execution, i.e. has already committed to allowing L2
to perform I/O.  If L1 (or host userspace) modifies the I/O permission
bitmaps during the exit to userspace,  KVM will treat the access as being
intercepted despite already having emulated the I/O access.

Pivot on EMULTYPE_NO_DECODE to detect that KVM is completing emulation.
Of the three users of EMULTYPE_NO_DECODE, only complete_emulated_io() (the
intended "recipient") can reach the code in question.  gp_interception()'s
use is mutually exclusive with is_guest_mode(), and
complete_emulated_insn_gp() unconditionally pairs EMULTYPE_NO_DECODE with
EMULTYPE_SKIP.

The bad behavior was detected by a syzkaller program that toggles port I/O
interception during the userspace I/O exit, ultimately resulting in a WARN
on vcpu->arch.pio.count being non-zero due to KVM no completing emulation
of the I/O instruction.

  WARNING: CPU: 23 PID: 1083 at arch/x86/kvm/x86.c:8039 emulator_pio_in_out+0x154/0x170 [kvm]
  Modules linked in: kvm_intel kvm irqbypass
  CPU: 23 UID: 1000 PID: 1083 Comm: repro Not tainted 6.16.0-rc5-c1610d2d66b1-next-vm #74 NONE
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  RIP: 0010:emulator_pio_in_out+0x154/0x170 [kvm]
  PKRU: 55555554
  Call Trace:
   <TASK>
   kvm_fast_pio+0xd6/0x1d0 [kvm]
   vmx_handle_exit+0x149/0x610 [kvm_intel]
   kvm_arch_vcpu_ioctl_run+0xda8/0x1ac0 [kvm]
   kvm_vcpu_ioctl+0x244/0x8c0 [kvm]
   __x64_sys_ioctl+0x8a/0xd0
   do_syscall_64+0x5d/0xc60
   entry_SYSCALL_64_after_hwframe+0x4b/0x53
   </TASK>

The Linux kernel CVE team has assigned CVE-2025-40026 to this issue.


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

	Issue introduced in 3.0 with commit 8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9 and fixed in 5.15.195 with commit e0ce3ed1048a47986d15aef1a98ebda25560d257
	Issue introduced in 3.0 with commit 8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9 and fixed in 6.1.157 with commit ba35a5d775799ce5ad60230be97336f2fefd518e
	Issue introduced in 3.0 with commit 8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9 and fixed in 6.6.111 with commit 3d3abf3f7e8b1abb082070a343de82d7efc80523
	Issue introduced in 3.0 with commit 8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9 and fixed in 6.12.52 with commit e7177c7e32cb806f348387b7f4faafd4a5b32054
	Issue introduced in 3.0 with commit 8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9 and fixed in 6.16.12 with commit 3a062a5c55adc5507600b9ae6d911e247e2f1d6e
	Issue introduced in 3.0 with commit 8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9 and fixed in 6.17.2 with commit 7366830642505683bbe905a2ba5d18d6e4b512b8
	Issue introduced in 3.0 with commit 8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9 and fixed in 6.18-rc1 with commit e750f85391286a4c8100275516973324b621a269

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-2025-40026
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/kvm/emulate.c
	arch/x86/kvm/kvm_emulate.h
	arch/x86/kvm/x86.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/e0ce3ed1048a47986d15aef1a98ebda25560d257
	https://git.kernel.org/stable/c/ba35a5d775799ce5ad60230be97336f2fefd518e
	https://git.kernel.org/stable/c/3d3abf3f7e8b1abb082070a343de82d7efc80523
	https://git.kernel.org/stable/c/e7177c7e32cb806f348387b7f4faafd4a5b32054
	https://git.kernel.org/stable/c/3a062a5c55adc5507600b9ae6d911e247e2f1d6e
	https://git.kernel.org/stable/c/7366830642505683bbe905a2ba5d18d6e4b512b8
	https://git.kernel.org/stable/c/e750f85391286a4c8100275516973324b621a269