On 10.12.2017 01:44, Wanpeng Li wrote: > 2017-12-08 20:39 GMT+08:00 David Hildenbrand <david@xxxxxxxxxx>: >> On 08.12.2017 10:12, 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 unconditionally before preempt/irq >>> enable. >>> >>> Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx> >>> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx> >>> Cc: Radim Krčmář <rkrcmar@xxxxxxxxxx> >>> Cc: David Hildenbrand <david@xxxxxxxxxx> >>> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx> >>> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx> >>> --- >>> 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. That make sense, as I assume this is the first time that a trap would be delivered. Reviewed-by: David Hildenbrand <david@xxxxxxxxxx> And certainly stable material? > > Regards, > Wanpeng Li > -- Thanks, David / dhildenb
