[3.8.y.z extended stable] Patch "kvm: x86: fix emulator buffer overflow (CVE-2014-0049)" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    kvm: x86: fix emulator buffer overflow (CVE-2014-0049)

to the linux-3.8.y-queue branch of the 3.8.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.8.y-queue

This patch is scheduled to be released in version 3.8.13.20.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.8.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 0684c1e2058d0fe06497e6312b664fdd0ec5a715 Mon Sep 17 00:00:00 2001
From: Andrew Honig <ahonig at google.com>
Date: Thu, 27 Feb 2014 19:35:14 +0100
Subject: kvm: x86: fix emulator buffer overflow (CVE-2014-0049)

commit a08d3b3b99efd509133946056531cdf8f3a0c09b upstream.

The problem occurs when the guest performs a pusha with the stack
address pointing to an mmio address (or an invalid guest physical
address) to start with, but then extending into an ordinary guest
physical address.  When doing repeated emulated pushes
emulator_read_write sets mmio_needed to 1 on the first one.  On a
later push when the stack points to regular memory,
mmio_nr_fragments is set to 0, but mmio_is_needed is not set to 0.

As a result, KVM exits to userspace, and then returns to
complete_emulated_mmio.  In complete_emulated_mmio
vcpu->mmio_cur_fragment is incremented.  The termination condition of
vcpu->mmio_cur_fragment == vcpu->mmio_nr_fragments is never achieved.
The code bounces back and fourth to userspace incrementing
mmio_cur_fragment past it's buffer.  If the guest does nothing else it
eventually leads to a a crash on a memcpy from invalid memory address.

However if a guest code can cause the vm to be destroyed in another
vcpu with excellent timing, then kvm_clear_async_pf_completion_queue
can be used by the guest to control the data that's pointed to by the
call to cancel_work_item, which can be used to gain execution.

Fixes: f78146b0f9230765c6315b2e14f56112513389ad
Signed-off-by: Andrew Honig <ahonig at google.com>
Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 arch/x86/kvm/x86.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index bf8c370..6d5a54c 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5888,7 +5888,7 @@ static int complete_emulated_mmio(struct kvm_vcpu *vcpu)
 		frag->len -= len;
 	}

-	if (vcpu->mmio_cur_fragment == vcpu->mmio_nr_fragments) {
+	if (vcpu->mmio_cur_fragment >= vcpu->mmio_nr_fragments) {
 		vcpu->mmio_needed = 0;
 		if (vcpu->mmio_is_write)
 			return 1;
--
1.8.3.2