On Tue, 7 Apr 2020 23:40:58 -0700
Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote:
> Check that the resolved slot (somewhat confusingly named 'start') is a
> valid/allocated slot before doing the final comparison to see if the
> specified gfn resides in the associated slot. The resolved slot can be
> invalid if the binary search loop terminated because the search index
> was incremented beyond the number of used slots.
>
> This bug has existed since the binary search algorithm was introduced,
> but went unnoticed because KVM statically allocated memory for the max
> number of slots, i.e. the access would only be truly out-of-bounds if
> all possible slots were allocated and the specified gfn was less than
> the base of the lowest memslot. Commit 36947254e5f98 ("KVM: Dynamically
> size memslot array based on number of used slots") eliminated the "all
> possible slots allocated" condition and made the bug embarrasingly easy
> to hit.
>
> Fixes: 9c1a5d38780e6 ("kvm: optimize GFN to memslot lookup with large slots amount")
> Reported-by: syzbot+d889b59b2bb87d4047a2@xxxxxxxxxxxxxxxxxxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> ---
> include/linux/kvm_host.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 6d58beb65454..01276e3d01b9 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -1048,7 +1048,7 @@ search_memslots(struct kvm_memslots *slots, gfn_t gfn)
> start = slot + 1;
> }
>
> - if (gfn >= memslots[start].base_gfn &&
> + if (start < slots->used_slots && gfn >= memslots[start].base_gfn &&
> gfn < memslots[start].base_gfn + memslots[start].npages) {
> atomic_set(&slots->lru_slot, start);
> return &memslots[start];
Reviewed-by: Cornelia Huck <cohuck@xxxxxxxxxx>
