[PATCH v2 1/2] KVM: cpuid: Fix read/write out-of-bounds vulnerability in cpuid emulation

[Wanpeng Li <kernellwp@gmail.com>]

From: Wanpeng Li <wanpeng.li@hotmail.com>

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 the unused memory in host OS.

As Paolo pointed:

>>  	/* 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.
>
>> -			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 indeed we have:
>
>- cpuid_entries at offset 1B50 (6992)
>- maxphyaddr at offset 27D0 (6992 + 3200 = 10192)
>- padding at 27D4...27DF
>- emulate_ctxt at 27E0
>
>So this indeed 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. At 
the 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: Moguofang <moguofang@huawei.com>
Cc: stable@vger.kernel.org
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
v1 -> v2:
 * update patch description

 arch/x86/kvm/cpuid.c | 19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index a181ae7..b927a42 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -779,19 +779,20 @@ int kvm_dev_ioctl_get_cpuid(struct kvm_cpuid2 *cpuid,
 
 static int move_to_next_stateful_cpuid_entry(struct kvm_vcpu *vcpu, int i)
 {
+	int j = i, nent = vcpu->arch.cpuid_nent;
 	struct kvm_cpuid_entry2 *e = &vcpu->arch.cpuid_entries[i];
-	int j, nent = vcpu->arch.cpuid_nent;
+	struct kvm_cpuid_entry2 *ej;
 
 	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
-- 
2.7.4