rds: ib: reject FRMR registration before IB connection is established

rds_ib_get_mr() extracts the rds_ib_connection from conn->c_transport_data
and passes it to rds_ib_reg_frmr() for FRWR memory registration. On a
fresh outgoing connection, ic is allocated in rds_ib_conn_alloc() with
i_cm_id = NULL because the connection worker has not yet called
rds_ib_conn_path_connect() to create the rdma_cm_id. When sendmsg() with
RDS_CMSG_RDMA_MAP is called on such a connection, the sendmsg path parses
the control message before any connection establishment, allowing
rds_ib_post_reg_frmr() to dereference ic->i_cm_id->qp and crash the
kernel.

The existing guard in rds_ib_reg_frmr() only checks for !ic (added in
commit 9e630bc), which does not catch this case since ic is allocated
early and is always non-NULL once the connection object exists.

 KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]
 RIP: 0010:rds_ib_post_reg_frmr+0x50e/0x920
 Call Trace:
  rds_ib_post_reg_frmr (net/rds/ib_frmr.c:167)
  rds_ib_map_frmr (net/rds/ib_frmr.c:252)
  rds_ib_reg_frmr (net/rds/ib_frmr.c:430)
  rds_ib_get_mr (net/rds/ib_rdma.c:615)
  __rds_rdma_map (net/rds/rdma.c:295)
  rds_cmsg_rdma_map (net/rds/rdma.c:860)
  rds_sendmsg (net/rds/send.c:1363)
  ____sys_sendmsg
  do_syscall_64

Add a check in rds_ib_get_mr() that verifies ic, i_cm_id, and qp are all
non-NULL before proceeding with FRMR registration, mirroring the guard
already present in rds_ib_post_inv(). Return -ENODEV when the connection
is not ready, which the existing error handling in rds_cmsg_send() converts
to -EAGAIN for userspace retry and triggers rds_conn_connect_if_down() to
start the connection worker.

Fixes: 1659185 ("RDS: IB: Support Fastreg MR (FRMR) memory registration mode")
Reported-by: Xiang Mei <xmei5@asu.edu>
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
Reviewed-by: Allison Henderson <achender@kernel.org>
Link: https://patch.msgid.link/20260330163237.2752440-2-bestswngs@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: winmin

net/rds/ib_rdma.c

@@ -604,8 +604,13 @@ void *rds_ib_get_mr(struct scatterlist *sg, unsigned long nents,
 		return ibmr;
 	}
 
-	if (conn)
+	if (conn) {
 		ic = conn->c_transport_data;
+		if (!ic || !ic->i_cm_id || !ic->i_cm_id->qp) {
+			ret = -ENODEV;
+			goto out;
+		}
+	}
 
 	if (!rds_ibdev->mr_8k_pool || !rds_ibdev->mr_1m_pool) {
 		ret = -ENODEV;