2017-12-12 23:08 GMT+08:00 Paolo Bonzini <[email protected]>: > On 10/12/2017 01:44, Wanpeng Li wrote: >> 2017-12-08 20:39 GMT+08:00 David Hildenbrand <[email protected]>: >>> On 08.12.2017 10:12, Wanpeng Li wrote: >>>> From: Wanpeng Li <[email protected]> >>>> >>>> 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 <[email protected]> >>>> Cc: Paolo Bonzini <[email protected]> >>>> Cc: Radim Krčmář <[email protected]> >>>> Cc: David Hildenbrand <[email protected]> >>>> Cc: Dmitry Vyukov <[email protected]> >>>> Signed-off-by: Wanpeng Li <[email protected]> >>>> --- >>>> 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
