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

[Wanpeng Li]

2017-12-08 20:39 GMT+08:00 David Hildenbrand <david@xxxxxxxxxx>:
> On 08.12.2017 10:12, Wanpeng Li wrote:
>> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>>
>> 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@xxxxxxxxxx>
>> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
>> Cc: David Hildenbrand <david@xxxxxxxxxx>
>> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
>> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>> ---
>>  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.

I think it's fine to restore dr6 before preempt/irq enable.

Regards,
Wanpeng Li