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

[Wanpeng Li]

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 KrÄmÃÅ <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());
 
-- 
2.7.4