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

From: Wanpeng Li
Date: Wed Dec 13 2017 - 04:53:15 EST


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