Patch "KVM: x86: fix escape of guest dr6 to the host" has been added to the 4.4-stable tree

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 4.4-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-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Wanpeng Li <[email protected]>
Date: Wed, 13 Dec 2017 10:46:40 +0100
Subject: KVM: x86: fix escape of guest dr6 to the host
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From: Wanpeng Li <[email protected]>

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.

---
 arch/x86/kvm/x86.c |    6 ++++++
 1 file changed, 6 insertions(+)

@@ -2755,6 +2755,12 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *
 	kvm_x86_ops->vcpu_put(vcpu);
 	kvm_put_guest_fpu(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 [email protected] are

queue-4.4/kvm-x86-fix-escape-of-guest-dr6-to-the-host.patch