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Message-ID: <YKvHFbPfGnaQ4huw@google.com>
Date: Mon, 24 May 2021 15:32:37 +0000
From: Sean Christopherson <seanjc@...gle.com>
To: Wanpeng Li <kernellwp@...il.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
Paolo Bonzini <pbonzini@...hat.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Wanpeng Li <wanpengli@...cent.com>,
Jim Mattson <jmattson@...gle.com>,
Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH] KVM: X86: Fix warning caused by stale emulation context
On Sun, May 23, 2021, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
>
> 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@...kaller.appspotmail.com
> Fixes: 4a1e10d5b5d8 (KVM: x86: handle hardware breakpoints during emulation)
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
> 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.
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 8a0ccdb56076..b62944046d7d 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -5322,7 +5322,8 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len)
ctxt->execute = opcode.u.execute;
- if (unlikely(ctxt->ud) && likely(!(ctxt->d & EmulateOnUD)))
+ if (unlikely(emulation_type & EMULTYPE_TRAP_UD) &&
+ likely(!(ctxt->d & EmulateOnUD)))
return EMULATION_FAILED;
if (unlikely(ctxt->d &
diff --git a/arch/x86/kvm/kvm_emulate.h b/arch/x86/kvm/kvm_emulate.h
index f016838faedd..2ad32600a8e3 100644
--- a/arch/x86/kvm/kvm_emulate.h
+++ b/arch/x86/kvm/kvm_emulate.h
@@ -314,7 +314,6 @@ struct x86_emulate_ctxt {
int interruptibility;
bool perm_ok; /* do not check permissions if true */
- bool ud; /* inject an #UD if host doesn't support insn */
bool tf; /* TF value before instruction (after for syscall/sysret) */
bool have_exception;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a224601d89e2..48b49c24c086 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7552,8 +7552,6 @@ int x86_decode_emulated_instruction(struct kvm_vcpu *vcpu, int emulation_type,
init_emulate_ctxt(vcpu);
- ctxt->ud = emulation_type & EMULTYPE_TRAP_UD;
-
/*
* We will reenter on the same instruction since we do not set
* complete_userspace_io. This does not handle watchpoints yet,
@@ -7563,7 +7561,7 @@ int x86_decode_emulated_instruction(struct kvm_vcpu *vcpu, int emulation_type,
kvm_vcpu_check_breakpoint(vcpu, &r))
return r;
- r = x86_decode_insn(ctxt, insn, insn_len);
+ r = x86_decode_insn(ctxt, insn, insn_len, emulation_type);
trace_kvm_emulate_insn_start(vcpu);
++vcpu->stat.insn_emulation;
> +
> /*
> * We will reenter on the same instruction since we do not set
> * complete_userspace_io. This does not handle watchpoints yet,
> @@ -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
>
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