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

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Date:   Wed, 13 Dec 2017 11:11:08 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     David Hildenbrand <david@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH] KVM: X86: Fix host dr6 miss restore

2017-12-12 23:08 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
> On 10/12/2017 01:44, Wanpeng Li wrote:
>> 2017-12-08 20:39 GMT+08:00 David Hildenbrand <david@...hat.com>:
>>> On 08.12.2017 10:12, Wanpeng Li wrote:
>>>> From: Wanpeng Li <wanpeng.li@...mail.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@...gle.com>
>>>> Cc: Paolo Bonzini <pbonzini@...hat.com>
>>>> Cc: Radim Krčmář <rkrcmar@...hat.com>
>>>> Cc: David Hildenbrand <david@...hat.com>
>>>> Cc: Dmitry Vyukov <dvyukov@...gle.com>
>>>> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.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.
>
> If no breakpoint is active, you should be able to restore it only in
> sched_out.

Do it in v2. :)

Regards,
Wanpeng Li