Re: [PATCH 6.1.y] KVM: x86: Don't (re)check L1 intercepts when completing userspace I/O

[Sean Christopherson <seanjc@google.com>]

On Mon, Oct 13, 2025, Sasha Levin wrote:
> From: Sean Christopherson <seanjc@google.com>
> 
> [ Upstream commit e750f85391286a4c8100275516973324b621a269 ]
> 
> 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>
> 
> Reported-by: syzbot+cc2032ba16cc2018ca25@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/68790db4.a00a0220.3af5df.0020.GAE@google.com
> Fixes: 8a76d7f25f8f ("KVM: x86: Add x86 callback for intercept check")
> Cc: stable@vger.kernel.org
> Cc: Jim Mattson <jmattson@google.com>
> Link: https://lore.kernel.org/r/20250715190638.1899116-1-seanjc@google.com
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> [ is_guest_mode() was open coded ]
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---

Acked-by: Sean Christopherson <seanjc@google.com>

Thanks Sasha!