Patch "KVM: x86: fix escape of guest dr6 to the host" has been added to the 4.15-stable tree

[<gregkh@linuxfoundation.org>]

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;a=summary

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