KVM: x86: fix for buffer overflow in handling of MSR_KVM_SYSTEM_TIME (CVE-2013-1796)

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@google.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f7c850b36910..2ade60c25402 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1959,6 +1959,11 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
                 /* ...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);