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. -- Thanks, David / dhildenb
