Subject: Re: [PATCH] ipv6: udp: fix memory leak in udpv6_sendmsg error path

From: Sabrina Dubroca <[email-masked]>
Date: Wed, 22 Apr 2026 13:55:44 +0200

2026-04-22, 18:58:02 +0800, Mingyu Wang wrote:
> During fuzzing with failslab enabled, a memory leak was observed in the
> IPv6 UDP send path.
> 
> When sending via the lockless fast path (!corkreq), udpv6_sendmsg()
> calls ip6_make_skb() and assumes that the routing entry (dst_entry)
> reference has been stolen by the callee. However, if ip6_make_skb()
> fails early (e.g., due to an ENOMEM from memory allocation failure),
> it returns an error pointer without consuming the dst reference.

Not in all cases? If ip6_setup_cork() fails, we call
ip6_cork_release() which will release the dst. The MSG_PROBE path also
releases the dst. __ip6_flush_pending_frames() also looks like it does
that.

> Since udpv6_sendmsg() unconditionally jumps to the 'out_no_dst' label,
> the unconsumed dst_entry is never released, resulting in a memory leak.
> 
> Fix this by explicitly calling dst_release(dst) when ip6_make_skb()
> returns an error.
> 
> Signed-off-by: Mingyu Wang <[email-masked]>

And this is missing a Fixes tag.

>  net/ipv6/udp.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
> index 15e032194ecc..b83ecfd729af 100644
> --- a/net/ipv6/udp.c
> +++ b/net/ipv6/udp.c
> @@ -1706,8 +1706,11 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
>  				   dst_rt6_info(dst),
>  				   msg->msg_flags, &cork);
>  		err = PTR_ERR(skb);
> -		if (!IS_ERR_OR_NULL(skb))
> +		if (!IS_ERR_OR_NULL(skb)) {
>  			err = udp_v6_send_skb(skb, fl6, &cork.base);
> +		} else {
> +			dst_release(dst);
> +		}
>  		/* ip6_make_skb steals dst reference */

This comment becomes really confusing after your patch.

>  		goto out_no_dst;
>  	}
> -- 
> 2.34.1

-- 
Sabrina