From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
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@xxxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Radim Krčmář <rkrcmar@xxxxxxxxxx>
Cc: Moguofang <moguofang@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
---
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