Re: [PATCH v2] ipv6: fix memory leak in __ip6_make_skb() when queue is empty

[syzbot <syzbot@syzkaller.appspotmail.com>]

> During fuzzing with failslab enabled, a memory leak was observed in the
> IPv6 UDP send path.
>
> The root cause resides in __ip6_make_skb(). In extremely rare cases
> (such as fault injection or specific empty payload conditions),
> __ip6_append_data() may succeed but leave the socket's write queue
> empty.
>
> When __ip6_make_skb() is subsequently called, __skb_dequeue(queue)
> returns NULL. The previous logic handled this by executing a 'goto out;',
> which completely bypassed the call to ip6_cork_release(cork).
>
> Since the 'cork' structure actively holds a reference to the routing
> entry (dst_entry) and potentially other allocated options, skipping
> the release cleanly leaks these resources.
>
> Fix this by introducing an 'out_cork_release' label and jumping to it
> when skb is NULL, ensuring the cork state is always properly cleaned up.
> The now-unused 'out' label is also removed to prevent compiler warnings.
>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Reported-by: syzbot+e5d6936b9f4545fd88ab@syzkaller.appspotmail.com
> Signed-off-by: Mingyu Wang <25181214217@stu.xidian.edu.cn>
> ---
>  net/ipv6/ip6_output.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
> index 7e92909ab5be..82210dd5eb96 100644
> --- a/net/ipv6/ip6_output.c
> +++ b/net/ipv6/ip6_output.c
> @@ -1934,7 +1934,7 @@ struct sk_buff *__ip6_make_skb(struct sock *sk,
>  
>  	skb = __skb_dequeue(queue);
>  	if (!skb)
> -		goto out;
> +		goto out_cork_release;
>  	tail_skb = &(skb_shinfo(skb)->frag_list);
>  
>  	/* move skb->data to ip header from ext header */
> @@ -1998,8 +1998,8 @@ struct sk_buff *__ip6_make_skb(struct sock *sk,
>  		ICMP6_INC_STATS(net, idev, ICMP6_MIB_OUTMSGS);
>  	}
>  
> +out_cork_release:
>  	ip6_cork_release(cork);
> -out:
>  	return skb;
>  }
>  
> -- 
> 2.34.1
>

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