Re: [PATCH v2] skb_expand_head() adjust skb->truesize incorrectly

[Eric Dumazet]

On 8/29/21 5:59 AM, Vasily Averin wrote:
> Christoph Paasch reports [1] about incorrect skb->truesize
> after skb_expand_head() call in ip6_xmit.
> This may happen because of two reasons:
> - skb_set_owner_w() for newly cloned skb is called too early,
> before pskb_expand_head() where truesize is adjusted for (!skb-sk) case.
> - pskb_expand_head() does not adjust truesize in (skb->sk) case.
> In this case sk->sk_wmem_alloc should be adjusted too.
> 
> [1] https://lkml.org/lkml/2021/8/20/1082
> 
> Reported-by: Christoph Paasch <christoph.paasch@xxxxxxxxx>
> Signed-off-by: Vasily Averin <vvs@xxxxxxxxxxxxx>
> ---
> v2: based on patch version from Eric Dumazet,
>     added __pskb_expand_head() function, which can be forced
>     to adjust skb->truesize and sk->sk_wmem_alloc.
> ---
>  net/core/skbuff.c | 43 +++++++++++++++++++++++++++++--------------
>  1 file changed, 29 insertions(+), 14 deletions(-)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index f931176..4691023 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -1681,10 +1681,10 @@ struct sk_buff *__pskb_copy_fclone(struct sk_buff *skb, int headroom,
>   *	reloaded after call to this function.
>   */
>  
> -int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
> -		     gfp_t gfp_mask)
> +static int __pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
> +			      gfp_t gfp_mask, bool update_truesize)
>  {
> -	int i, osize = skb_end_offset(skb);
> +	int delta, i, osize = skb_end_offset(skb);
>  	int size = osize + nhead + ntail;
>  	long off;
>  	u8 *data;
> @@ -1756,9 +1756,13 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
>  	 * For the moment, we really care of rx path, or
>  	 * when skb is orphaned (not attached to a socket).
>  	 */
> -	if (!skb->sk || skb->destructor == sock_edemux)
> -		skb->truesize += size - osize;
> -
> +	delta = size - osize;
> +	if (!skb->sk || skb->destructor == sock_edemux) {
> +		skb->truesize += delta;
> +	} else if (update_truesize) {

Unfortunately we can not always do this sk_wmem_alloc change here.

Some skb have skb->sk set, but the 'reference on socket' is not through sk_wmem_alloc

It seems you need a helper to make sure skb->destructor is one of
the destructors that use skb->truesize and sk->sk_wmem_alloc

For instance, skb_orphan_partial() could have been used.



> +		refcount_add(delta, &skb->sk->sk_wmem_alloc);
> +		skb->truesize += delta;
> +	}
>  	return 0;
>  
>  nofrags:
> @@ -1766,6 +1770,12 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
>  nodata:
>  	return -ENOMEM;
>  }
> +
> +int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
> +		     gfp_t gfp_mask)
> +{
> +	return __pskb_expand_head(skb, nhead, ntail, gfp_mask, false);
> +}
>  EXPORT_SYMBOL(pskb_expand_head);
>  
>  /* Make private copy of skb with writable head and some headroom */
> @@ -1804,28 +1814,33 @@ struct sk_buff *skb_realloc_headroom(struct sk_buff *skb, unsigned int headroom)
>  struct sk_buff *skb_expand_head(struct sk_buff *skb, unsigned int headroom)
>  {
>  	int delta = headroom - skb_headroom(skb);
> +	struct sk_buff *oskb = NULL;
>  
>  	if (WARN_ONCE(delta <= 0,
>  		      "%s is expecting an increase in the headroom", __func__))
>  		return skb;
>  
> +	delta = SKB_DATA_ALIGN(delta);
>  	/* pskb_expand_head() might crash, if skb is shared */
>  	if (skb_shared(skb)) {
>  		struct sk_buff *nskb = skb_clone(skb, GFP_ATOMIC);
>  
> -		if (likely(nskb)) {
> -			if (skb->sk)
> -				skb_set_owner_w(nskb, skb->sk);
> -			consume_skb(skb);
> -		} else {
> +		if (unlikely(!nskb)) {
>  			kfree_skb(skb);
> +			return NULL;
>  		}
> +		oskb = skb;
>  		skb = nskb;
>  	}
> -	if (skb &&
> -	    pskb_expand_head(skb, SKB_DATA_ALIGN(delta), 0, GFP_ATOMIC)) {
> +	if (__pskb_expand_head(skb, delta, 0, GFP_ATOMIC, true)) {
>  		kfree_skb(skb);
> -		skb = NULL;
> +		kfree_skb(oskb);
> +		return NULL;
> +	}
> +	if (oskb) {
> +		if (oskb->sk)
> +			skb_set_owner_w(skb, oskb->sk);
> +		consume_skb(oskb);
>  	}
>  	return skb;
>  }
>