Re: [PATCH net-next 06/12] Revert "net: ptr_ring: otherwise safe empty checks can overrun array bounds"

[John Fastabend]

On 01/25/2018 03:36 PM, Michael S. Tsirkin wrote:
> This reverts commit bcecb4bbf88aa03171c30652bca761cf27755a6b.
> 
> If we try to allocate an extra entry as the above commit did, and when
> the requested size is UINT_MAX, addition overflows causing zero size to
> be passed to kmalloc().
> 
> kmalloc then returns ZERO_SIZE_PTR with a subsequent crash.
> 
> Reported-by: syzbot+87678bcf753b44c39b67@xxxxxxxxxxxxxxxxxxxxxxxxx
> Cc: John Fastabend <john.fastabend@xxxxxxxxx>
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> ---

Dang, I missed this case. Thanks.

Acked-by: John Fastabend <john.fastabend@xxxxxxxxx>

>  include/linux/ptr_ring.h | 7 +------
>  1 file changed, 1 insertion(+), 6 deletions(-)
> 
> diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
> index f175846..3a19ebd 100644
> --- a/include/linux/ptr_ring.h
> +++ b/include/linux/ptr_ring.h
> @@ -466,12 +466,7 @@ static inline int ptr_ring_consume_batched_bh(struct ptr_ring *r,
>  
>  static inline void **__ptr_ring_init_queue_alloc(unsigned int size, gfp_t gfp)
>  {
> -	/* Allocate an extra dummy element at end of ring to avoid consumer head
> -	 * or produce head access past the end of the array. Possible when
> -	 * producer/consumer operations and __ptr_ring_peek operations run in
> -	 * parallel.
> -	 */
> -	return kcalloc(size + 1, sizeof(void *), gfp);
> +	return kcalloc(size, sizeof(void *), gfp);
>  }
>  
>  static inline void __ptr_ring_set_size(struct ptr_ring *r, int size)
>