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

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Date:   Tue, 12 Dec 2017 16:08:25 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>,
        David Hildenbrand <david@...hat.com>
Cc:     "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

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.

Paolo