[PATCH net-next v2] net: rds: check cmsg_len before reading rds_rdma_args in size pass

[Michael Bommarito <michael.bommarito@gmail.com>]

rds_rm_size() handles RDS_CMSG_RDMA_ARGS after only CMSG_OK() and then
calls rds_rdma_extra_size(), which reads args->local_vec_addr and
args->nr_local without first checking that cmsg_len covers struct
rds_rdma_args. The other two RDS_CMSG_RDMA_ARGS consumers already guard
this: rds_rdma_bytes() in rds_sendmsg() and rds_cmsg_rdma_args() in
rds_cmsg_send() both reject cmsg_len < CMSG_LEN(sizeof(struct
rds_rdma_args)). Add the same check to rds_rm_size() so all three RDMA
args passes are consistent.

This is a consistency and hardening change with no behavioral effect for
well-formed senders and no reachable bug today: rds_rdma_bytes() runs
before rds_rm_size() in rds_sendmsg() and already rejects a short
RDS_CMSG_RDMA_ARGS, so the size pass is not reached with an undersized
cmsg. But rds_rm_size() reads the args independently of that earlier
pass, and nothing in rds_rm_size() itself records or enforces the
precondition, so a reader or a future refactor of the size pass cannot
tell the cmsg has already been length-checked. Applying the same
cmsg_len guard in all three RDS_CMSG_RDMA_ARGS consumers keeps that
invariant local to each and robust to reordering.

Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
v2:
 - Re-target net-next and drop the Fixes: tag and the stable Cc. This
   is a consistency/hardening change, not a reachable bug: as Allison
   Henderson noted, rds_rdma_bytes() runs before rds_rm_size() in
   rds_sendmsg() and already rejects a short RDS_CMSG_RDMA_ARGS, so a
   user cannot reach the rds_rm_size() read through sendmsg.
 - Corrected the changelog: the two sibling guards are rds_rdma_bytes()
   in rds_sendmsg() and rds_cmsg_rdma_args() in rds_cmsg_send(); the
   former runs before, not after, rds_rm_size().
 - Dropped the KASAN/AF_RDS reachability framing. No code change from v1.
 - v1: https://lore.kernel.org/all/20260614130725.2520842-1-michael.bommarito@gmail.com/

 net/rds/send.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/rds/send.c b/net/rds/send.c
index d8b14ff9d366b..6ca3192b1d8af 100644
--- a/net/rds/send.c
+++ b/net/rds/send.c
@@ -967,6 +967,8 @@ static int rds_rm_size(struct msghdr *msg, int num_sgs,
 
 		switch (cmsg->cmsg_type) {
 		case RDS_CMSG_RDMA_ARGS:
+			if (cmsg->cmsg_len < CMSG_LEN(sizeof(struct rds_rdma_args)))
+				return -EINVAL;
 			if (vct->indx >= vct->len) {
 				vct->len += vct->incr;
 				tmp_iov =

base-commit: 5200f5f493f79f14bbdc349e402a40dfb32f23c8
-- 
2.53.0