Re: [PATCH] KVM: X86: Fix warning caused by stale emulation context

[Wanpeng Li]

On Mon, 24 May 2021 at 23:32, Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Sun, May 23, 2021, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> >
> > Reported by syzkaller:
> >
> >   WARNING: CPU: 7 PID: 10526 at /home/kernel/ssd/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 4a1e10d5b5d8c (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 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 before hardware breakpoints check.
> >
> > 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>
> > ---
> >  arch/x86/kvm/x86.c | 14 +++++++-------
> >  1 file changed, 7 insertions(+), 7 deletions(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index bbc4e04..eca69f9 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -7552,6 +7552,13 @@ int x86_decode_emulated_instruction(struct kvm_vcpu *vcpu, int emulation_type,
> >
> >       init_emulate_ctxt(vcpu);
> >
> > +     ctxt->interruptibility = 0;
> > +     ctxt->have_exception = false;
> > +     ctxt->exception.vector = -1;
> > +     ctxt->perm_ok = false;
>
> What about moving this block all the way into init_emulate_ctxt()?
>
> > +     ctxt->ud = emulation_type & EMULTYPE_TRAP_UD;
>
> This can be left where it is since ctxt->ud is consumed only by x86_decode_insn().
> I don't have a strong preference as it really only matters for the backport.  For
> upstream, we can kill it off in a follow-up patch by passing emulation_type to
> x86_decode_insn() and dropping ctxt->ud altogether.  Tracking that info in ctxt
> for literally one call is silly.

Good suggestion, will do in v2.

    Wanpeng