net/rds: zero per-item info buffer before handing it to visitors

rds_for_each_conn_info() and rds_walk_conn_path_info() both hand a
caller-allocated on-stack u64 buffer to a per-connection visitor and
then copy the full item_len bytes back to user space via
rds_info_copy() regardless of how much of the buffer the visitor
actually wrote.

rds_ib_conn_info_visitor() and rds6_ib_conn_info_visitor() only
write a subset of their output struct when the underlying
rds_connection is not in state RDS_CONN_UP (src/dst addr, tos, sl
and the two GIDs via explicit memsets). Several u32 fields
(max_send_wr, max_recv_wr, max_send_sge, rdma_mr_max, rdma_mr_size,
cache_allocs) and the 2-byte alignment hole between sl and
cache_allocs remain as whatever stack contents preceded the visitor
call and are then memcpy_to_user()'d out to user space.

struct rds_info_rdma_connection and struct rds6_info_rdma_connection
are the only rds_info_* structs in include/uapi/linux/rds.h that are
not marked __attribute__((packed)), so they have a real alignment
hole. The other info visitors (rds_conn_info_visitor,
rds6_conn_info_visitor, rds_tcp_tc_info, ...) write all fields of
their packed output struct today and are not known to be vulnerable,
but a future visitor that adds a conditional write-path would have
the same bug.

Reproduction on a kernel built without CONFIG_INIT_STACK_ALL_ZERO=y:
a local unprivileged user opens AF_RDS, sets SO_RDS_TRANSPORT=IB,
binds to a local address on an RDMA-capable netdev (rxe soft-RoCE on
any netdev is sufficient), sendto()'s any peer on the same subnet
(fails cleanly but installs an rds_connection in the global hash in
RDS_CONN_CONNECTING), then calls getsockopt(SOL_RDS,
RDS_INFO_IB_CONNECTIONS). The returned 68-byte item contains 26
bytes of stack garbage including kernel text/data pointers:

    0..7   0a 63 00 01 0a 63 00 02     src=10.99.0.1 dst=10.99.0.2
    8..39  00 ...                      gids (memset-zeroed)
    40..47 e0 92 a3 81 ff ff ff ff     kernel pointer (max_send_wr)
    48..55 7f 37 b5 81 ff ff ff ff     kernel pointer (rdma_mr_max)
    56..59 01 00 08 00                 rdma_mr_size (garbage)
    60..61 00 00                       tos, sl
    62..63 00 00                       alignment padding
    64..67 18 00 00 00                 cache_allocs (garbage)

Fix by zeroing the per-item buffer in both rds_for_each_conn_info()
and rds_walk_conn_path_info() before invoking the visitor. This
covers the IPv4/IPv6 IB visitors and hardens all current and future
visitors against the same class of bug.

No functional change for visitors that fully populate their output.

Changes in v2:
- retarget at the net tree (subject prefix "[PATCH net v2]",
  net/rds: prefix in the title)
- pick up Reviewed-by tags from Sharath Srinivasan and
  Allison Henderson

Fixes: ec16227 ("RDS/IB: Infiniband transport")
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Reviewed-by: Sharath Srinivasan <sharath.srinivasan@oracle.com>
Reviewed-by: Allison Henderson <achender@kernel.org>
Assisted-by: Claude:claude-opus-4-7
Link: https://patch.msgid.link/20260418141047.3398203-1-michael.bommarito@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: mjbommar

net/rds/connection.c

@@ -701,6 +701,13 @@ void rds_for_each_conn_info(struct socket *sock, unsigned int len,
 	     i++, head++) {
 		hlist_for_each_entry_rcu(conn, head, c_hash_node) {
 
+			/* Zero the per-item buffer before handing it to the
+			 * visitor so any field the visitor does not write -
+			 * including implicit alignment padding - cannot leak
+			 * stack contents to user space via rds_info_copy().
+			 */
+			memset(buffer, 0, item_len);
+
 			/* XXX no c_lock usage.. */
 			if (!visitor(conn, buffer))
 				continue;
@@ -750,6 +757,13 @@ static void rds_walk_conn_path_info(struct socket *sock, unsigned int len,
 			 */
 			cp = conn->c_path;
 
+			/* Zero the per-item buffer for the same reason as
+			 * rds_for_each_conn_info(): any byte the visitor
+			 * does not write (including alignment padding) must
+			 * not leak stack contents via rds_info_copy().
+			 */
+			memset(buffer, 0, item_len);
+
 			/* XXX no cp_lock usage.. */
 			if (!visitor(cp, buffer))
 				continue;