Re: [PATCH net 1/2] net: check the sendmsg LSM hook for every batched message

[Kuniyuki Iwashima <kuniyu@google.com>]

On Sat, 31 Oct 2026 13:08:40 +0200, Avi Weiss wrote:
> sendmmsg() skips security_socket_sendmsg() when two consecutive
> messages have the same explicit destination address. The comparison
> does not account for the message size, so a BPF LSM policy that denies
> large sends can be bypassed by placing an allowed message first.

What is the real use case ?

or is this something just flagged by AI ?


> 
> Use __sock_sendmsg() for the cached-destination case as well, so that
> every message undergoes the LSM check. Keep the remainder of the
> address-caching logic unchanged in this minimal correction.
> 
> Fixes: 228e548e6020 ("net: Add sendmmsg socket system call")

This is net-next material.


> Signed-off-by: Avi Weiss <thnkslprpt@gmail.com>
> ---
>  net/socket.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/socket.c b/net/socket.c
> index 2e3b5737c481..c2a85224ab55 100644
> --- a/net/socket.c
> +++ b/net/socket.c
> @@ -2709,7 +2709,7 @@ static int ____sys_sendmsg(struct socket *sock, struct msghdr *msg_sys,

 	/*
 	 * If this is sendmmsg() and current destination address is same as
 	 * previously succeeded address, omit asking LSM's decision.
 	 * used_address->name_len is initialized to UINT_MAX so that the first
 	 * destination address never matches.
 	 */

The comment here is stale.

 	if (used_address && msg_sys->msg_name &&

>  	    used_address->name_len == msg_sys->msg_namelen &&
>  	    !memcmp(&used_address->name, msg_sys->msg_name,
>  		    used_address->name_len)) {

Also, used_address, etc is no longer needed.


> -		err = sock_sendmsg_nosec(sock, msg_sys);
> +		err = __sock_sendmsg(sock, msg_sys);
>  		goto out_freectl;
>  	}
>  	err = __sock_sendmsg(sock, msg_sys);
> -- 
> 2.43.0
>