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

[David Hildenbrand <david@redhat.com>]

On 08.12.2017 10:12, 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 unconditionally before preempt/irq 
> enable.
> 
> 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>
> Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
> ---
>  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 0c5d55c..a6370fd 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7065,6 +7065,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  	 */
>  	if (hw_breakpoint_active())
>  		hw_breakpoint_restore();
> +	else
> +		set_debugreg(current->thread.debugreg6, 6);
>  
>  	vcpu->arch.last_guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
>  
> 

If you haven't seen it, I analyzed this in
https://lkml.org/lkml/2017/11/7/638 but nobody would respond for now to
my suggestion/question.

-- 

Thanks,

David / dhildenb