KVM: x86: work around leak of uninitialized stack contents (CVE-2019-7222)

Bugzilla: 1671930

Emulation of certain instructions (VMXON, VMCLEAR, VMPTRLD, VMWRITE with
memory operand, INVEPT, INVVPID) can incorrectly inject a page fault
when passed an operand that points to an MMIO address.  The page fault
will use uninitialized kernel stack memory as the CR2 and error code.

The right behavior would be to abort the VM with a KVM_EXIT_INTERNAL_ERROR
exit to userspace; however, it is not an easy fix, so for now just
ensure that the error code and CR2 are zero.

Embargoed until Feb 7th 2019.

Reported-by: Felix Wilhelm <fwilhelm@google.com>
Cc: stable@kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 3d27206f6c010b13d408d45a4eb647430bf2e50d..e67ecf25e69087ff6aa8711001a59b175477a3b1 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5116,6 +5116,13 @@ int kvm_read_guest_virt(struct kvm_vcpu *vcpu,
 {
        u32 access = (kvm_x86_ops->get_cpl(vcpu) == 3) ? PFERR_USER_MASK : 0;
 
+       /*
+        * FIXME: this should call handle_emulation_failure if X86EMUL_IO_NEEDED
+        * is returned, but our callers are not ready for that and they blindly
+        * call kvm_inject_page_fault.  Ensure that they at least do not leak
+        * uninitialized kernel stack memory into cr2 and error code.
+        */
+       memset(exception, 0, sizeof(*exception));
        return kvm_read_guest_virt_helper(addr, val, bytes, vcpu, access,
                                          exception);
 }