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());
--
2.7.4
