Re: [PATCH v3 1/2] KVM: X86: Fix warning caused by stale emulation context

[Sean Christopherson]

On Wed, May 26, 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]

"/home/kernel/ssd/linux/" can be omitted to make the line length a bit shorter.
checkpatch also complains about using absolute path instead of relative path.

>   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 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.  
> 
> 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>
> ---
> v2 -> v3:
>  * squash ctxt->ud
> v1 -> v2:
>  * move the second part emulation context initialization into init_emulate_ctxt()
> 
>  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..ae47b19 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7226,6 +7226,13 @@ static void init_emulate_ctxt(struct kvm_vcpu *vcpu)
>  	BUILD_BUG_ON(HF_SMM_MASK != X86EMUL_SMM_MASK);
>  	BUILD_BUG_ON(HF_SMM_INSIDE_NMI_MASK != X86EMUL_SMM_INSIDE_NMI_MASK);
>  
> +	ctxt->interruptibility = 0;
> +	ctxt->have_exception = false;
> +	ctxt->exception.vector = -1;
> +	ctxt->perm_ok = false;
> +
> +	ctxt->ud = emulation_type & EMULTYPE_TRAP_UD;

"ctxt->ud" should be left where it is in patch 01.  "emulation_type" isn't passed
to init_emulate_ctxt(), and I don't see any reason to add it to the params since
ctxt->ud is only consumed by x86_decode_insn(), i.e. moving ctxt->ud isn't
necessary to fix the bug.

arch/x86/kvm/x86.c: In function ‘init_emulate_ctxt’:
arch/x86/kvm/x86.c:7236:13: error: ‘emulation_type’ undeclared (first use in this function); did you mean ‘exception_type’?
 7236 |  ctxt->ud = emulation_type & EMULTYPE_TRAP_UD;
      |             ^~~~~~~~~~~~~~
      |             exception_type
arch/x86/kvm/x86.c:7236:13: note: each undeclared identifier is reported only once for each function it appears in

> +
>  	init_decode_cache(ctxt);
>  	vcpu->arch.emulate_regs_need_sync_from_vcpu = false;
>  }
> @@ -7561,13 +7568,6 @@ int x86_decode_emulated_instruction(struct kvm_vcpu *vcpu, int emulation_type,
>  	    kvm_vcpu_check_breakpoint(vcpu, &r))
>  		return r;
>  
> -	ctxt->interruptibility = 0;
> -	ctxt->have_exception = false;
> -	ctxt->exception.vector = -1;
> -	ctxt->perm_ok = false;
> -
> -	ctxt->ud = emulation_type & EMULTYPE_TRAP_UD;
> -
>  	r = x86_decode_insn(ctxt, insn, insn_len);
>  
>  	trace_kvm_emulate_insn_start(vcpu);
> -- 
> 2.7.4
>