Re: [PATCH net-next v4 12/12] net/rds: hold a connection reference from struct rds_incoming

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- [Medium] The commit message's reachability claim — "reaching the use-
  after-free requires freeing a connection out from under a live socket,…
- [Low] The patch closes a reported, KASAN-confirmed use-after-free
  (`rds6_inc_info_copy()` reading a freed `rds_connection` via…

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net/rds: hold a connection reference from struct rds_incoming

This makes every assignment of rds_incoming->i_conn take a connection
reference, dropped from rds_inc_put() and from the new rds_message_free()
helper, so a connection stays allocated while incs that arrived over it
are still queued on socket receive queues.

Closes: https://lore.kernel.org/netdev/20260720184955.3008978-1-nicoyip.dev@gmail.com/ (local)

The two comments below are about the changelog only, so the diff hunks
are snipped.

    This is not a stable candidate: reaching the use-after-free requires
    freeing a connection out from under a live socket, which needs
    CAP_SYS_MODULE, netns teardown or physical device removal, and the fix
    depends on the connection reference counting introduced earlier in this
    series.

[Severity: Medium]
Is this list of prerequisites complete?  There looks to be a fourth way
to free a live connection that needs none of the three, in
rds_ib_cm_connect_complete():

net/rds/ib_cm.c:rds_ib_cm_connect_complete() {
	dp = event->param.conn.private_data;
	...
		major = dp->ricp_v4.dp_protocol_major;
		minor = dp->ricp_v4.dp_protocol_minor;
	...
	if (major) {
		rds_ib_set_protocol(conn, RDS_PROTOCOL(major, minor));
	...
	if (conn->c_version < RDS_PROTOCOL_VERSION) {
		if (conn->c_version != RDS_PROTOCOL_COMPAT_VERSION) {
			pr_notice("RDS/IB: Connection <%pI6c,%pI6c> version %u.%u no longer supported\n", ...);
			rds_conn_destroy(conn);
			return;
		}
	}
}

The version comes from the peer's CM private data, and this callback is
reached from live event handling:

rds_rdma_cm_event_handler_cmn() -> RDMA_CM_EVENT_ESTABLISHED
  -> trans->cm_connect_complete()
    -> rds_ib_cm_connect_complete()
      -> rds_conn_destroy()

Since the same struct rds_connection is reused across reconnects for a
given laddr/faddr/tos tuple, can a reconnect on which the peer advertises
an unsupported version destroy a connection that unread incs from the
previous session still point at, without CAP_SYS_MODULE, netns teardown or
device removal?

If so, could the paragraph be reworded to cover that path, and the stable
reasoning re-derived from it?  The dependency on the reference counting
introduced earlier in the series is a separate argument.

One related note: the sibling patch "net/rds: pin the connection across
RDMA-CM event handling" states this call "has meanwhile been switched to
rds_conn_drop() by commit f97d8c7bab78", but at this revision
rds_ib_cm_connect_complete() still calls rds_conn_destroy(conn).  Which of
the two is right?

    Reported-by: Chengfeng Ye [off-list ref]
    Closes: https://lore.kernel.org/netdev/20260720184955.3008978-1-nicoyip.dev@gmail.com/ (local)

[Severity: Low]
Should this carry a Fixes: tag for the commit that introduced the bare
i_conn assignment in rds_inc_init()?

The changelog documents two user-visible crashes, the KASAN report in
rds6_inc_info_copy() and the panic in rds_ib_inc_free(), and the second
one is reachable on the pre-series code, where nothing keeps the
connection alive for a queued inc:

net/rds/ib_recv.c:rds_ib_inc_free() {
	struct rds_ib_connection *ic = inc->i_conn->c_transport_data;
	...
	rds_ib_recv_cache_put(&ibinc->ii_cache_entry, &ic->i_cache_incs);
}

reached by rds_release() -> rds_clear_recv_queue() -> rds_inc_put().

The other patches in this series that fix pre-existing defects do carry
one (e266df6b -> Fixes: 745cbccac3fe, 23531807 -> Fixes: 1c5113cf796b,
d4f3ede0 -> Fixes: ebeeb1ad9b8a, ef753cfe -> Fixes: cdc306a5c9cd3), so
this one looks inconsistent with the rest.  Even if no backport is
possible because the fix depends on the earlier reference counting, would
naming the introducing commit, or stating which kernels are exposed, help
downstream trees decide whether they are affected?

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