Re: [PATCH net 1/2] ipv4: fix IP ID reuse in ip_select_ident_segs()

[Jiayuan Chen <jiayuan.chen@linux.dev>]

On 9/29/26 9:12 PM, Eric Dumazet wrote:
> ip_select_ident_segs() must put the first of the @segs reserved IP IDs
> in iph->id, as GSO assigns id, id + 1, ..., id + segs - 1 to segments.
>
> Commit f866fbc842de ("ipv4: fix data-races around inet->inet_id")
> used atomic_add_return() for non-TCP sockets, which returns the first
> ID of the next packet instead. Consecutive GSO packets can then reuse
> IP IDs.
>
> SCTP GSO is affected. Other callers use segs == 1, and only see
> a harmless off-by-one (UDP GSO has a separate, older issue, see
> following patch in this series).
>
> Use atomic_fetch_add() instead, like the TCP path.
>
> Fixes: f866fbc842de ("ipv4: fix data-races around inet->inet_id")
> Signed-off-by: Eric Dumazet <edumazet@kernel.org>



Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>


> ---
>   include/net/ip.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/net/ip.h b/include/net/ip.h
> index 6f602df72ee621ee4ee45e70beef0a1b5145367f..6a3e8271a73b3669e97c4b389e916dbcbfa9f6ae 100644
> --- a/include/net/ip.h
> +++ b/include/net/ip.h
> @@ -598,7 +598,7 @@ static inline void ip_select_ident_segs(struct net *net, struct sk_buff *skb,
>   			val = atomic_read(&inet_sk(sk)->inet_id);
>   			atomic_set(&inet_sk(sk)->inet_id, val + segs);
>   		} else {
> -			val = atomic_add_return(segs, &inet_sk(sk)->inet_id);
> +			val = atomic_fetch_add(segs, &inet_sk(sk)->inet_id);
>   		}
>   		iph->id = htons(val);
>   		return;