KVM: x86: work around leak of uninitialized stack contents

Emulation of VMPTRST 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.
Signed-off-by: Fuqian Huang <huangfq.daxian@gmail.com>
Cc: stable@vger.kernel.org
[add comment]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -5312,6 +5312,13 @@ int kvm_write_guest_virt_system(struct kvm_vcpu *vcpu, gva_t addr, void *val,
 	/* kvm_write_guest_virt_system can pull in tons of pages. */
 	vcpu->arch.l1tf_flush_l1d = true;
 
+	/*
+	 * 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_write_guest_virt_helper(addr, val, bytes, vcpu,
 					   PFERR_WRITE_MASK, exception);
 }