diff options
| author | Wanpeng Li <wanpeng.li@hotmail.com> | 2017-12-13 10:46:40 +0100 |
|---|---|---|
| committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2018-02-25 11:15:40 +0100 |
| commit | a009a6388ceaf409f508886adb6d1db08caddf02 (patch) | |
| tree | f60965e112b67a9416047a63a73fbad9214748ba /arch | |
| parent | 53712962126d8be88fd9cc80c64a3cf71628c74e (diff) | |
KVM: x86: fix escape of guest dr6 to the host
commit efdab992813fb2ed825745625b83c05032e9cda2 upstream.
syzkaller reported:
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 testcase sets a watchpoint (with perf_event_open) on a buffer that is
passed to timer_create() as the struct sigevent argument. In timer_create(),
copy_from_user()'s rep movsb triggers the BP. The testcase also sets
the debug registers for the guest.
However, KVM only restores host debug registers when the host has active
watchpoints, which triggers a race condition when running the testcase with
multiple threads. The guest's DR6.BS bit can escape to the host before
another thread invokes timer_create(), and do_debug() complains.
The fix is to respect do_debug()'s dr6 invariant when leaving KVM.
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>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'arch')
| -rw-r--r-- | arch/x86/kvm/x86.c | 6 |
1 files changed, 6 insertions, 0 deletions
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index ac381437c291..17f4eca37d22 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -2939,6 +2939,12 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu) pagefault_enable(); kvm_x86_ops->vcpu_put(vcpu); vcpu->arch.last_host_tsc = rdtsc(); + /* + * If userspace has set any breakpoints or watchpoints, dr6 is restored + * on every vmexit, but if not, we might have a stale dr6 from the + * guest. do_debug expects dr6 to be cleared after it runs, do the same. + */ + set_debugreg(0, 6); } static int kvm_vcpu_ioctl_get_lapic(struct kvm_vcpu *vcpu, |