On 08/06/2017 10:22, Wanpeng Li wrote:
> 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
Queued, thanks.
Paolo
>
> 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
>