[ 3.5.y.z extended stable ] Patch "KVM: x86: fix for buffer overflow in handling of" has been added to staging queue

Luis Henriques luis.henriques at canonical.com
Thu Apr 11 09:09:41 UTC 2013

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

    KVM: x86: fix for buffer overflow in handling of

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:


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

For more information about the 3.5.y.z tree, see



>From 9ebdfca37460b564bc58ca0a50a477fba06250e0 Mon Sep 17 00:00:00 2001
From: Andy Honig <ahonig at google.com>
Date: Mon, 11 Mar 2013 09:34:52 -0700
Subject: [PATCH] KVM: x86: fix for buffer overflow in handling of

commit c300aa64ddf57d9c5d9c898a64b36877345dd4a9 upstream.

If the guest sets the GPA of the time_page so that the request to update the
time straddles a page then KVM will write onto an incorrect page.  The
write is done byusing kmap atomic to get a pointer to the page for the time
structure and then performing a memcpy to that page starting at an offset
that the guest controls.  Well behaved guests always provide a 32-byte aligned
address, however a malicious guest could use this to corrupt host kernel

Tested: Tested against kvmclock unit test.

Signed-off-by: Andrew Honig <ahonig at google.com>
Signed-off-by: Marcelo Tosatti <mtosatti at redhat.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
 arch/x86/kvm/x86.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 8b6fbee..ad5cf4b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1618,6 +1618,11 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 data)
 		/* ...but clean it before doing the actual write */
 		vcpu->arch.time_offset = data & ~(PAGE_MASK | 1);

+		/* Check that the address is 32-byte aligned. */
+		if (vcpu->arch.time_offset &
+				(sizeof(struct pvclock_vcpu_time_info) - 1))
+			break;
 		vcpu->arch.time_page =
 				gfn_to_page(vcpu->kvm, data >> PAGE_SHIFT);


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