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

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Message-ID: <CANRm+CydkdwCJqqm4OfgSMJmaxJzhDG6Td5LO3YTbUC=4Ht8Lw@mail.gmail.com>
Date:   Wed, 13 Dec 2017 17:53:06 +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 v2] KVM: X86: Fix host dr6 miss restore

2017-12-13 17:42 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
> On 13/12/2017 10:18, David Hildenbrand wrote:
>> On 13.12.2017 04:10, 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 in sched_out if no breakpoint is active.
>>>
>>> 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>
>>> Reviewed-by: David Hildenbrand <david@...hat.com>
>>> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.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.

Do you need I to send a new version?

Regards,
Wanpeng Li