linux-kernel - [PATCH] KVM: VMX: downgrade warning on unexpected exit code

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Date:   Fri, 13 Jan 2017 18:59:04 +0100
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        KVM list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Steve Rutherford <srutherford@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>
Subject: [PATCH] KVM: VMX: downgrade warning on unexpected exit code

2017-01-02 11:23+0100, Dmitry Vyukov:
> Hello,
> 
> I've got the following warning while running syzkaller fuzzer:
> 
> WARNING: CPU: 2 PID: 13257 at arch/x86/kvm/vmx.c:8633
> vmx_handle_exit+0x262b/0x38b0 arch/x86/kvm/vmx.c:8633
> vmx: unexpected exit reason 0xb
> CPU: 2 PID: 13257 Comm: syz-executor7 Not tainted 4.10.0-rc1+ #118
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> Call Trace:
>  __dump_stack lib/dump_stack.c:15 [inline]
>  dump_stack+0x292/0x3a2 lib/dump_stack.c:51
>  panic+0x1cb/0x3a9 kernel/panic.c:179
>  __warn+0x1c4/0x1e0 kernel/panic.c:539
>  warn_slowpath_fmt+0xc5/0x100 kernel/panic.c:562
>  vmx_handle_exit+0x262b/0x38b0 arch/x86/kvm/vmx.c:8633
>  vcpu_enter_guest arch/x86/kvm/x86.c:6884 [inline]
>  vcpu_run arch/x86/kvm/x86.c:6943 [inline]
>  kvm_arch_vcpu_ioctl_run+0xf3d/0x45f0 arch/x86/kvm/x86.c:7101
>  kvm_vcpu_ioctl+0x673/0x1120 arch/x86/kvm/../../../virt/kvm/kvm_main.c:2569
>  vfs_ioctl fs/ioctl.c:43 [inline]
>  do_vfs_ioctl+0x1bf/0x1780 fs/ioctl.c:683
>  SYSC_ioctl fs/ioctl.c:698 [inline]
>  SyS_ioctl+0x8f/0xc0 fs/ioctl.c:689
>  entry_SYSCALL_64_fastpath+0x1f/0xc2
> RIP: 0033:0x4438a9
> RSP: 002b:00007f935aa2fb58 EFLAGS: 00000286 ORIG_RAX: 0000000000000010
> RAX: ffffffffffffffda RBX: 0000000000000015 RCX: 00000000004438a9
> RDX: 0000000000000000 RSI: 000000000000ae80 RDI: 0000000000000015
> RBP: 00000000006ddb30 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000286 R12: 0000000000700000
> R13: 0000000000000006 R14: 0000000000000015 R15: 0000000020014000
> Dumping ftrace buffer:
>    (ftrace buffer empty)
> Kernel Offset: disabled
> 
> Exit reason 0xb is GETSEC instruction. This does not look harmful as
> it is handled as #UD. But I think we should print a single line
> message regarding non-emulated instruction as in other cases, just to
> not scare cloud admins and to make syzkaller ignore it.
> 
> FTR, a raw reproducer is here:
> https://gist.githubusercontent.com/dvyukov/c762f6ea04ebbba49cdee0a6caca31b4/raw/f21deb04cdc70ae74100c12447d71bb0cd2025c7/gistfile1.txt

I can't reproduce -- maybe a nested bug, which will take a while to
figure out.  Still, host dump at that point is useless, so the change
makes sense.

A guest dump would be useful (e.g. I think that GETSEC should not exit
if guest CR4.SMX is disabled), but just giving the error is about as
good and we want something short and rate-limited if the message can be
trigerred by a guest in production ...

---8<---
We never needed the call trace and we better rate-limit if it can be
triggered by a guest.

Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
---
 arch/x86/kvm/vmx.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index a236decb81e4..7cd606daa01e 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -8595,7 +8595,8 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu)
 	    && kvm_vmx_exit_handlers[exit_reason])
 		return kvm_vmx_exit_handlers[exit_reason](vcpu);
 	else {
-		WARN_ONCE(1, "vmx: unexpected exit reason 0x%x\n", exit_reason);
+		vcpu_unimpl(vcpu, "vmx: unexpected exit reason 0x%x\n",
+				exit_reason);
 		kvm_queue_exception(vcpu, UD_VECTOR);
 		return 1;
 	}
-- 
2.11.0