[PATCH] KVM: x86: fix escape of guest dr6 to the host

[PATCH] KVM: x86: fix escape of guest dr6 to the host

[Paolo Bonzini]

syzkaller reported:

   WARNING: CPU: 0 PID: 12927 at arch/x86/kernel/traps.c:780 do_debug+0x222/0x250
   CPU: 0 PID: 12927 Comm: syz-executor Tainted: G           OE    4.15.0-rc2+ #16
   RIP: 0010:do_debug+0x222/0x250
   Call Trace:
    <#DB>
    debug+0x3e/0x70
   RIP: 0010:copy_user_enhanced_fast_string+0x10/0x20
    </#DB>
    _copy_from_user+0x5b/0x90
    SyS_timer_create+0x33/0x80
    entry_SYSCALL_64_fastpath+0x23/0x9a

The testcase sets a watchpoint (with perf_event_open) on a buffer that is
passed to timer_create() as the struct sigevent argument.  In timer_create(),
copy_from_user()'s rep movsb triggers the BP.  The testcase also sets
the debug registers for the guest.

However, KVM only restores host debug registers when the host has active
watchpoints, which triggers a race condition when running the testcase with
multiple threads.  The guest's DR6.BS bit can escape to the host before
another thread invokes timer_create(), and do_debug() complains.

The fix is to respect do_debug()'s dr6 invariant when leaving KVM.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
 arch/x86/kvm/x86.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 9212dad176a7..eed73217e96f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2951,6 +2951,12 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
 	pagefault_enable();
 	kvm_x86_ops->vcpu_put(vcpu);
 	vcpu->arch.last_host_tsc = rdtsc();
+	/*
+	 * If userspace has set any breakpoints or watchpoints, dr6 is restored
+	 * on every vmexit, but if not, we might have a stale dr6 from the
+	 * guest. do_debug expects dr6 to be cleared after it runs, do the same.
+	 */
+	set_debugreg(0, 6);
 }
 
 static int kvm_vcpu_ioctl_get_lapic(struct kvm_vcpu *vcpu,
-- 
1.8.3.1

Re: [PATCH] KVM: x86: fix escape of guest dr6 to the host

[Wanpeng Li]

2017-12-13 17:53 GMT+08:00 Paolo Bonzini <pbonzini@redhat.com>:
> syzkaller reported:
>
>    WARNING: CPU: 0 PID: 12927 at arch/x86/kernel/traps.c:780 do_debug+0x222/0x250
>    CPU: 0 PID: 12927 Comm: syz-executor Tainted: G           OE    4.15.0-rc2+ #16
>    RIP: 0010:do_debug+0x222/0x250
>    Call Trace:
>     <#DB>
>     debug+0x3e/0x70
>    RIP: 0010:copy_user_enhanced_fast_string+0x10/0x20
>     </#DB>
>     _copy_from_user+0x5b/0x90
>     SyS_timer_create+0x33/0x80
>     entry_SYSCALL_64_fastpath+0x23/0x9a
>
> The testcase sets a watchpoint (with perf_event_open) on a buffer that is
> passed to timer_create() as the struct sigevent argument.  In timer_create(),
> copy_from_user()'s rep movsb triggers the BP.  The testcase also sets
> the debug registers for the guest.
>
> However, KVM only restores host debug registers when the host has active
> watchpoints, which triggers a race condition when running the testcase with
> multiple threads.  The guest's DR6.BS bit can escape to the host before
> another thread invokes timer_create(), and do_debug() complains.
>
> The fix is to respect do_debug()'s dr6 invariant when leaving KVM.
>
> Reported-by: Dmitry Vyukov <dvyukov@google.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Reviewed-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>

Thanks for the help! :)

Regards,
Wanpeng Li

> ---
>  arch/x86/kvm/x86.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 9212dad176a7..eed73217e96f 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2951,6 +2951,12 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>         pagefault_enable();
>         kvm_x86_ops->vcpu_put(vcpu);
>         vcpu->arch.last_host_tsc = rdtsc();
> +       /*
> +        * If userspace has set any breakpoints or watchpoints, dr6 is restored
> +        * on every vmexit, but if not, we might have a stale dr6 from the
> +        * guest. do_debug expects dr6 to be cleared after it runs, do the same.
> +        */
> +       set_debugreg(0, 6);
>  }
>
>  static int kvm_vcpu_ioctl_get_lapic(struct kvm_vcpu *vcpu,
> --
> 1.8.3.1
>