[PATCH v2] KVM: X86: Fix host dr6 miss restore

[Wanpeng Li <kernellwp@gmail.com>]

From: Wanpeng Li <wanpeng.li@hotmail.com>

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@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
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();
+	if (!hw_breakpoint_active())
+		set_debugreg(current->thread.debugreg6, 6);
 }
 
 static int kvm_vcpu_ioctl_get_lapic(struct kvm_vcpu *vcpu,
-- 
2.7.4