Re: [PATCH net] net/rds: ib: drop fragments shorter than the header-declared length

[Allison Henderson]

On Tue, 2026-10-06 at 18:27 +0530, Shubham Antil wrote:
> rds_ib_process_recv() accepts an incoming RDS/IB fragment once the
> receive completion reports at least an RDS header
> (data_len >= sizeof(struct rds_header)).  It then trusts the
> header-declared total message length h_len: for the first fragment of
> a message it stores be32_to_cpu(hdr->h_len) in ic->i_recv_data_rem,
> and rds_ib_inc_copy_to_user() later copies up to h_len bytes from the
> fragment pages to userspace on recvmsg().
> 
> The number of payload bytes actually received into the fragment page is
> data_len (after subtracting the header), but it is never checked against
> the amount the fragment is accounted to contribute to the message,
> min(i_recv_data_rem, RDS_FRAG_SIZE).  A fragment whose header advertises
> a larger h_len than the payload it delivers is still linked onto the
> reassembly list.  The fragment page comes from the per-CPU receive cache
> and is not zeroed, so rds_ib_inc_copy_to_user() then copies up to h_len
> bytes to the PF_RDS reader, including the uninitialized tail the receive
> never wrote.
> 
> Reject a fragment that carries fewer payload bytes than it is accounted
> to contribute before linking it onto the reassembly list.
> 
> The issue is reproducible under KMSAN with two hosts over rdma_rxe
> (Soft-RoCE); the same reproducer confirms the fix stops it.
> 
> Fixes: 1e23b3ee0e94 ("RDS/IB: Receive datagrams via IB")
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Shubham Antil <shubham@octane.security>

Hi Shubham,

Thanks for working on this.  This patch looks good to me, you can add my rvb:
Reviewed-by: Allison Henderson <achender@xxxxxxxxxx>

Also, this bug was found and reported privately by another contributor, whom I had
counseled to send a patch publicly before I had noticed this one.  
https://lore.kernel.org/netdev/20261006205204.1322102-1-tjdqudcks0424@xxxxxxxxx/

I find that patches equivalent, and this patch was posted first. But I would like to apply
the reported by tag since sungbyeongchan was the first to report it.
Reported-by: sungbyeongchan <tjdqudcks0424@xxxxxxxxx>

Thank you both for working on this bug!
Allison

> ---
>  net/rds/ib_recv.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/net/rds/ib_recv.c b/net/rds/ib_recv.c
> index bd6cb3ffa..fee77b6d7 100644
> --- a/net/rds/ib_recv.c
> +++ b/net/rds/ib_recv.c
> @@ -949,6 +949,15 @@ static void rds_ib_process_recv(struct rds_connection *conn,
>  		}
>  	}
>  
> +	/* h_len must be backed by the payload actually received (data_len),
> +	 * else the unwritten frag-page tail is copied to userspace.
> +	 */
> +	if (data_len < min_t(u32, ic->i_recv_data_rem, RDS_FRAG_SIZE)) {
> +		rds_ib_conn_error(conn,
> +			"fragment shorter than header-declared length; forcing reconnect\n");
> +		goto done;
> +	}
> +
>  	list_add_tail(&recv->r_frag->f_item, &ibinc->ii_frags);
>  	recv->r_frag = NULL;
>