KVM: cpuid: Fix read/write out-of-bounds vulnerability in cpuid emulation

If "i" is the last element in the vcpu->arch.cpuid_entries[] array, it
potentially can be exploited the vulnerability. this will out-of-bounds
read and write.  Luckily, the effect is small:

	/* when no next entry is found, the current entry[i] is reselected */
	for (j = i + 1; ; j = (j + 1) % nent) {
		struct kvm_cpuid_entry2 *ej = &vcpu->arch.cpuid_entries[j];
		if (ej->function == e->function) {

It reads ej->maxphyaddr, which is user controlled.  However...

			ej->flags |= KVM_CPUID_FLAG_STATE_READ_NEXT;

After cpuid_entries there is

	int maxphyaddr;
	struct x86_emulate_ctxt emulate_ctxt;  /* 16-byte aligned */

So we have:

- cpuid_entries at offset 1B50 (6992)
- maxphyaddr at offset 27D0 (6992 + 3200 = 10192)
- padding at 27D4...27DF
- emulate_ctxt at 27E0

And it writes in the padding.  Pfew, writing the ops field of emulate_ctxt
would have been much worse.

This patch fixes it by modding the index to avoid the out-of-bounds
access. Worst case, i == j and ej->function == e->function,
the loop can bail out.

Reported-by: Moguofang <moguofang@huawei.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Guofang Mo <moguofang@huawei.com>
Cc: stable@vger.kernel.org
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index a181ae76c71ce102f88f6a4ee9cc3233677c55e2..59ca2eea522c466937287c3e660b8f38e49c12aa 100644 (file)
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -780,18 +780,20 @@ out:
 static int move_to_next_stateful_cpuid_entry(struct kvm_vcpu *vcpu, int i)
 {
        struct kvm_cpuid_entry2 *e = &vcpu->arch.cpuid_entries[i];
-       int j, nent = vcpu->arch.cpuid_nent;
+       struct kvm_cpuid_entry2 *ej;
+       int j = i;
+       int nent = vcpu->arch.cpuid_nent;
 
        e->flags &= ~KVM_CPUID_FLAG_STATE_READ_NEXT;
        /* when no next entry is found, the current entry[i] is reselected */
-       for (j = i + 1; ; j = (j + 1) % nent) {
-               struct kvm_cpuid_entry2 *ej = &vcpu->arch.cpuid_entries[j];
-               if (ej->function == e->function) {
-                       ej->flags |= KVM_CPUID_FLAG_STATE_READ_NEXT;
-                       return j;
-               }
-       }
-       return 0; /* silence gcc, even though control never reaches here */
+       do {
+               j = (j + 1) % nent;
+               ej = &vcpu->arch.cpuid_entries[j];
+       } while (ej->function != e->function);
+
+       ej->flags |= KVM_CPUID_FLAG_STATE_READ_NEXT;
+
+       return j;
 }
 
 /* find an entry with matching function, matching index (if needed), and that