Subject: Re: [PATCH net] ptr_ring: use kmalloc_array()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 25 Aug 2017 21:03:17 +0300

On Wed, Aug 16, 2017 at 10:36:47AM -0700, Eric Dumazet wrote:
> From: Eric Dumazet <[email-masked]>
> 
> As found by syzkaller, malicious users can set whatever tx_queue_len
> on a tun device and eventually crash the kernel.
> 
> Lets remove the ALIGN(XXX, SMP_CACHE_BYTES) thing since a small
> ring buffer is not fast anyway.

I'm not sure it's worth changing for small rings.

Does kmalloc_array guarantee cache line alignment for big buffers
then? If the ring is misaligned it will likely cause false sharing
as it's designed to be accessed from two CPUs.

> Fixes: 2e0ab8ca83c1 ("ptr_ring: array based FIFO for pointers")
> Signed-off-by: Eric Dumazet <[email-masked]>
> Reported-by: Dmitry Vyukov <[email-masked]>
> Cc: Michael S. Tsirkin <[email-masked]>
> Cc: Jason Wang <[email-masked]>
> ---
>  include/linux/ptr_ring.h  |    9 +++++----
>  include/linux/skb_array.h |    3 ++-
>  2 files changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
> index d8c97ec8a8e6..37b4bb2545b3 100644
> --- a/include/linux/ptr_ring.h
> +++ b/include/linux/ptr_ring.h
> @@ -436,9 +436,9 @@ static inline int ptr_ring_consume_batched_bh(struct ptr_ring *r,
>  	__PTR_RING_PEEK_CALL_v; \
>  })
>  
> -static inline void **__ptr_ring_init_queue_alloc(int size, gfp_t gfp)
> +static inline void **__ptr_ring_init_queue_alloc(unsigned int size, gfp_t gfp)
>  {
> -	return kzalloc(ALIGN(size * sizeof(void *), SMP_CACHE_BYTES), gfp);
> +	return kcalloc(size, sizeof(void *), gfp);
>  }
>  
>  static inline void __ptr_ring_set_size(struct ptr_ring *r, int size)
> @@ -582,7 +582,8 @@ static inline int ptr_ring_resize(struct ptr_ring *r, int size, gfp_t gfp,
>   * In particular if you consume ring in interrupt or BH context, you must
>   * disable interrupts/BH when doing so.
>   */
> -static inline int ptr_ring_resize_multiple(struct ptr_ring **rings, int nrings,
> +static inline int ptr_ring_resize_multiple(struct ptr_ring **rings,
> +					   unsigned int nrings,
>  					   int size,
>  					   gfp_t gfp, void (*destroy)(void *))
>  {
> @@ -590,7 +591,7 @@ static inline int ptr_ring_resize_multiple(struct ptr_ring **rings, int nrings,
>  	void ***queues;
>  	int i;
>  
> -	queues = kmalloc(nrings * sizeof *queues, gfp);
> +	queues = kmalloc_array(nrings, sizeof(*queues), gfp);
>  	if (!queues)
>  		goto noqueues;
>  
> diff --git a/include/linux/skb_array.h b/include/linux/skb_array.h
> index 35226cd4efb0..8621ffdeecbf 100644
> --- a/include/linux/skb_array.h
> +++ b/include/linux/skb_array.h
> @@ -193,7 +193,8 @@ static inline int skb_array_resize(struct skb_array *a, int size, gfp_t gfp)
>  }
>  
>  static inline int skb_array_resize_multiple(struct skb_array **rings,
> -					    int nrings, int size, gfp_t gfp)
> +					    int nrings, unsigned int size,
> +					    gfp_t gfp)
>  {
>  	BUILD_BUG_ON(offsetof(struct skb_array, ring));
>  	return ptr_ring_resize_multiple((struct ptr_ring **)rings,
>