Re: [PATCH v4 1/2] KVM: X86: Fix warning caused by stale emulation context

[Sean Christopherson]

On Thu, May 27, 2021, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> 
> Reported by syzkaller:
> 
>   WARNING: CPU: 7 PID: 10526 at linux/arch/x86/kvm//x86.c:7621 x86_emulate_instruction+0x41b/0x510 [kvm]
>   RIP: 0010:x86_emulate_instruction+0x41b/0x510 [kvm]
>   Call Trace:
>    kvm_mmu_page_fault+0x126/0x8f0 [kvm]
>    vmx_handle_exit+0x11e/0x680 [kvm_intel]
>    vcpu_enter_guest+0xd95/0x1b40 [kvm]
>    kvm_arch_vcpu_ioctl_run+0x377/0x6a0 [kvm]
>    kvm_vcpu_ioctl+0x389/0x630 [kvm]
>    __x64_sys_ioctl+0x8e/0xd0
>    do_syscall_64+0x3c/0xb0
>    entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> Commit 4a1e10d5b5d8 ("KVM: x86: handle hardware breakpoints during emulation())
> adds hardware breakpoints check before emulation the instruction and parts of 
> emulation context initialization, actually we don't have the EMULTYPE_NO_DECODE flag 
> here and the emulation context will not be reused. Commit c8848cee74ff ("KVM: x86:
> set ctxt->have_exception in x86_decode_insn()) triggers the warning because it 
> catches the stale emulation context has #UD, however, it is not during instruction 
> decoding which should result in EMULATION_FAILED. This patch fixes it by moving 
> the second part emulation context initialization into init_emulate_ctxt() and 
> before hardware breakpoints check. The ctxt->ud will be dropped by a follow-up 
> patch.
> 
> syzkaller source: https://syzkaller.appspot.com/x/repro.c?x=134683fdd00000
> 
> Reported-by: syzbot+71271244f206d17f6441@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: 4a1e10d5b5d8 (KVM: x86: handle hardware breakpoints during emulation)
> Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> ---

Reviewed-by: Sean Christopherson <seanjc@xxxxxxxxxx>