This is a note to let you know that I've just added the patch titled
KVM: x86: fix escape of guest dr6 to the host
to the 4.15-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...
The filename of the patch is: kvm-x86-fix-escape-of-guest-dr6-to-the-host.patch and it can be found in the queue-4.15 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.
From efdab992813fb2ed825745625b83c05032e9cda2 Mon Sep 17 00:00:00 2001
From: Wanpeng Li wanpeng.li@hotmail.com Date: Wed, 13 Dec 2017 10:46:40 +0100 Subject: KVM: x86: fix escape of guest dr6 to the host MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit
From: Wanpeng Li wanpeng.li@hotmail.com
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
--- arch/x86/kvm/x86.c | 6 ++++++ 1 file changed, 6 insertions(+)
--- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -2939,6 +2939,12 @@ void kvm_arch_vcpu_put(struct kvm_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,
Patches currently in stable-queue which might be from wanpeng.li@hotmail.com are
queue-4.15/kvm-x86-fix-escape-of-guest-dr6-to-the-host.patch