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

[Paolo Bonzini <pbonzini@redhat.com>]

On 13/12/2017 10:18, David Hildenbrand wrote:
> 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.
> */

Actually, we should make it unconditionally zero, not reset it to
current->thread.debugreg6.  That's because the invariant at exit from
do_debug is DR6 = 0.

	/*
	 * do_debug expects dr6 to be cleared after it runs, but here
	 * we might have a stale dr6 from the guest.
	 */
	set_debugreg(0, 6);

I'll push the patch to kvm/queue.

Thanks,

Paolo