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 Krčmář <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. Thanks, Paolo
