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SubjectRe: [PATCH] KVM: X86: Fix host dr6 miss restore
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On 10.12.2017 01:44, Wanpeng Li wrote:
> 2017-12-08 20:39 GMT+08:00 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.
>
> I think it's fine to restore dr6 before preempt/irq enable.

That make sense, as I assume this is the first time that a trap would be
delivered.

Reviewed-by: David Hildenbrand <david@redhat.com>

And certainly stable material?

>
> Regards,
> Wanpeng Li
>


--

Thanks,

David / dhildenb

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