[1/2] KVM: Check validity of resolved slot when searching memslots
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Message ID 20200408064059.8957-2-sean.j.christopherson@intel.com
State New
Headers show
Series
  • KVM: Fix out-of-bounds memslot access
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Commit Message

Sean Christopherson April 8, 2020, 6:40 a.m. UTC
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@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---
 include/linux/kvm_host.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Cornelia Huck April 8, 2020, 7:04 a.m. UTC | #1
On Tue,  7 Apr 2020 23:40:58 -0700
Sean Christopherson <sean.j.christopherson@intel.com> 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@syzkaller.appspotmail.com
> Cc: stable@vger.kernel.org
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
>  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@redhat.com>

Patch
diff mbox series

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];