Patch "KVM: cpuid: Fix read/write out-of-bounds vulnerability in cpuid emulation" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

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

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
kvm-cpuid-fix-read-write-out-of-bounds-vulnerability-in-cpuid-emulation.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Wanpeng Li <[email protected]>
Date: Thu, 8 Jun 2017 01:22:07 -0700
Subject: KVM: cpuid: Fix read/write out-of-bounds vulnerability in cpuid emulation
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From: Wanpeng Li <[email protected]>

commit a3641631d14571242eec0d30c9faa786cbf52d44 upstream.

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.

---
 arch/x86/kvm/cpuid.c |   20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

@@ -765,18 +765,20 @@ 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

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/kvm-async_pf-fix-rcu_irq_enter-with-irqs-enabled.patch queue-4.9/kvm-async_pf-avoid-async-pf-injection-when-in-guest-mode.patch queue-4.9/kvm-cpuid-fix-read-write-out-of-bounds-vulnerability-in-cpuid-emulation.patch