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();
+ if (!hw_breakpoint_active())
+ set_debugreg(current->thread.debugreg6, 6);
}
static int kvm_vcpu_ioctl_get_lapic(struct kvm_vcpu *vcpu,
--
2.7.4
