[Lucid Precise Quantal Raring CVE-2013-1796] KVM: x86: fix for buffer overflow in handling of MSR_KVM_SYSTEM_TIME (CVE-2013-1796)

[Luis Henriques]

From: Andy Honig <ahonig at google.com>

CVE-2013-1796

BugLink: http://bugs.launchpad.net/bugs/1158254

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
memory.

Tested: Tested against kvmclock unit test.

Signed-off-by: Andrew Honig <ahonig at google.com>
Signed-off-by: Marcelo Tosatti <mtosatti at redhat.com>
(cherry picked from commit c300aa64ddf57d9c5d9c898a64b36877345dd4a9)

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 a52bd56..3fee182 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1635,6 +1635,11 @@ 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);
 
-- 
1.8.1.2