Re: [PATCH v2] KVM: X86: Fix host dr6 miss restore

[David Hildenbrand <david@redhat.com>]

On 13.12.2017 04:10, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@hotmail.com>
> 
> Reported by syzkaller:
> 
>    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 syzkaller will mmap a buffer which is also the struct sigevent parameter of
> timer_create(), it will also call perf_event_open() to set a BP for the buffer,
> so when the implementation of timer_create() in kernel tries to get the struct
> sigevent parameter by copy_from_user(), rep movsb triggers the BP. The syzkaller
> testcase also sets the debug registers for the guest, however, the kvm just
> restores host debug registers when we have active breakpoints. I can observe
> the dr6 single step bit is set and !hw_breakpoint_active() sporadically by print
> when running the testcase heavy multithreading. The do_debug() which is triggered
> by rep movsb will splash when (dr6 & DR_STEP && !user_mode(regs)).
> 
> This patch fixes it by restoring host dr6 in sched_out if no breakpoint is active.
> 
> 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>
> ---
> v1 -> v2:
>  * move to sched_out path
> 
>  arch/x86/kvm/x86.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 1c5c7a3..76886c4 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2964,6 +2964,8 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>  	pagefault_enable();
>  	kvm_x86_ops->vcpu_put(vcpu);
>  	vcpu->arch.last_host_tsc = rdtsc();

Can you add a comment like

/* With active breakpoints we already restored all debugregs in
   vcpu_enter_guest(), however without active breakpoints we have to
   restore debugreg 6 before scheduled out.
*/

> +	if (!hw_breakpoint_active())
> +		set_debugreg(current->thread.debugreg6, 6);
>  }
>  
>  static int kvm_vcpu_ioctl_get_lapic(struct kvm_vcpu *vcpu,
> 

Having the restore at the old place was for me easier to understands.
But this moves it out of the hot loop.

-- 

Thanks,

David / dhildenb