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

[Shubham Antil <shubham@octane.security>]

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>
---
 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;
 
-- 
2.43.0