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>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f7c850b36910154cf453dc4b91b2857454d0acdc..2ade60c254020c2f99fda1c6d35f2118d50a15da 100644 (file)
--- 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);