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Subject[PATCH 3.2 073/106] KVM: cpuid: Fix read/write out-of-bounds vulnerability in cpuid emulation
3.2.93-rc1 review patch.  If anyone has any objections, please let me know.

------------------

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

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.

Reported-by: Moguofang <moguofang@huawei.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Guofang Mo <moguofang@huawei.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[bwh: Backported to 3.2: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
arch/x86/kvm/x86.c | 20 +++++++++++---------
1 file changed, 11 insertions(+), 9 deletions(-)

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5450,18 +5450,20 @@ int emulator_fix_hypercall(struct x86_em
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
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