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. Regards, Wanpeng Li
