Re: [PATCH net-next v2 7/9] net/rds: pin the connection across RDMA-CM event handling

[netdev-bot+sashiko@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue(s) to consider:

Critical: 0 · High: 0 (2) · Medium: 0 · Low: 1

- [Low] Commit message misattributes a pre-existing use-after-free to the
  kref split, and never names the triggering callback.

Pre-existing issues:
- [High] Self-deadlock on the very callback path this patch relies on.
- [High] In the function this patch modifies, struct rds_transport
  *trans; (net/rds/rdma_transport.c:55) is assigned only under `if…

--

net/rds: pin the connection across RDMA-CM event handling

This adds a reference on the connection that rds_rdma_cm_event_handler_cmn()
picks up from cm_id->context, held for the duration of the handler, and skips
events for connections that are already being freed.  It also re-checks
rds_destroy_pending() under c_cm_lock in rds_ib_cm_handle_connect() so a
connect request cannot install a new cm_id and QP on a quiesced connection.

No Link: tags in the commit header.

A few questions below, mostly about the surrounding code that the new
reference is meant to protect.

First, on the changelog itself:

> Now that a connection is freed by its last reference rather than by
> rds_conn_destroy() itself, a callback that drops the last reference other
> than the handler's implicit one leaves the final mutex_unlock() running on
> freed memory.

[Severity: Low]

Is the hazard really new to this series?  In the baseline, rds_conn_destroy()
ended with:

	kfree(conn->c_path);
	kmem_cache_free(rds_conn_slab, conn);

so the connection was freed synchronously, and the handler's
mutex_unlock(&conn->c_cm_lock) at out: already ran on freed memory whenever a
callback destroyed the conn - unconditionally, not only when the last
reference happened to be dropped.

Also, which callback is meant here?  The only handler-reachable one seems to
be rds_ib_cm_connect_complete() on RDMA_CM_EVENT_ESTABLISHED, when the peer
advertises a protocol version below 3.1.  Could the changelog name that
trigger, so backporters know what to look for?

And what is "the handler's implicit one" reference?  Before this patch
cm_id->context carries no reference at all.

> diff --git a/net/rds/rdma_transport.c b/net/rds/rdma_transport.c
> index b15cf316b23a2..584e9867810f4 100644
> --- a/net/rds/rdma_transport.c
> +++ b/net/rds/rdma_transport.c
> @@ -63,6 +63,18 @@ static int rds_rdma_cm_event_handler_cmn(struct rdma_cm_id *cm_id,
>  	if (cm_id->device->node_type == RDMA_NODE_IB_CA)
>  		trans = &rds_ib_transport;
>  

[Severity: High]

This isn't a bug introduced by this patch, but while looking at these lines:
can trans stay uninitialized here?  In
rds_rdma_cm_event_handler_cmn() it is a plain local:

	struct rds_transport *trans;
	...
	if (cm_id->device->node_type == RDMA_NODE_IB_CA)
		trans = &rds_ib_transport;

There is no else branch and no early return, yet the CONNECT_REQUEST case
calls through it:

	case RDMA_CM_EVENT_CONNECT_REQUEST:
		ret = trans->cm_handle_connect(cm_id, event, isv6);

RDS never calls rdma_restrict_node_type(), so cma_listen_on_dev() keeps
restricted_node_type == RDMA_NODE_UNSPECIFIED and installs the RDS listener
on every RDMA device, including iWARP RNICs (siw, cxgb4, irdma, qedr and
erdma all set node_type = RDMA_NODE_RNIC).  An incoming connect on such a
device reaches iw_conn_req_handler(), which creates a child id inheriting the
listener's event_handler and NULL context, and delivers CONNECT_REQUEST here.

Since conn is NULL on that path, the new rds_conn_get_unless_zero() guard
does not change reachability, so this ends up as an indirect call through an
uninitialized stack slot driven by a remote peer.  Would an explicit else
that rejects non-IB node types be appropriate while touching this function?

> +	/* cm_id->context carries no reference of its own.  Pin the
> +	 * connection for the duration of the handler: what the callbacks
> +	 * below do may drop the last reference other than ours, and the
> +	 * mutex released at out: lives in the connection's path array.
> +	 * A connection already being freed gets no events handled.
> +	 */
> +	if (conn && !rds_conn_get_unless_zero(conn)) {
> +		rdsdebug("conn %p id %p is being freed, ignoring event\n",
> +			 conn, cm_id);
> +		return 0;
> +	}
> +

[Severity: High]

This is a pre-existing issue, not something this patch introduces, but it is
the exact callback path the comment above refers to: can the handler ever
return from that callback?

c_cm_lock is conn->c_path[0].cp_cm_lock, taken just below and held until
out:, and the ESTABLISHED case dispatches with it held:

	case RDMA_CM_EVENT_ESTABLISHED:
		if (conn)
			trans->cm_connect_complete(conn, event);

rds_ib_cm_connect_complete() takes the peer's protocol major/minor straight
from the REP private data with no validation, and since RDS_PROTOCOL_VERSION
and RDS_PROTOCOL_COMPAT_VERSION are both RDS_PROTOCOL_3_1, any advertised
version below 3.1 makes both tests true:

	if (conn->c_version < RDS_PROTOCOL_VERSION) {
		if (conn->c_version != RDS_PROTOCOL_COMPAT_VERSION) {
			...
			rds_conn_destroy(conn);

>From there the cycle looks like this:

rds_rdma_cm_event_handler_cmn()
  mutex_lock(&conn->c_cm_lock)
  trans->cm_connect_complete()
    rds_ib_cm_connect_complete()
      rds_conn_destroy()
        rds_conn_path_quiesce()
          rds_conn_path_drop(cp, true)   /* sets ERROR, queues cp_down_w */
          flush_work(&cp->cp_down_w)
            rds_shutdown_worker()
              rds_conn_shutdown()
                mutex_lock(&cp->cp_cm_lock)   /* held by the CM handler */

rds_conn_shutdown()'s first transition DOWN -> DOWN fails because
rds_conn_path_drop() left the state at RDS_CONN_ERROR, so it goes on to take
cp_cm_lock and blocks.  flush_work() then never returns, and the

	out:
		if (conn) {
			mutex_unlock(&conn->c_cm_lock);
			rds_conn_put(conn);
		}

that this patch adds is never reached, so the new pin can never be exercised
on this path.  The comment in rds_conn_shutdown() states the assumption that
is inverted here ("we may be deadlocking with the CM handler ... the CM event
handler is supposed to check for state DISCONNECTING") - here the CM handler
is the one starting the shutdown while holding the lock.

Should rds_ib_cm_connect_complete() defer the destroy (for example via
rds_conn_path_drop() without the synchronous flush) instead of calling
rds_conn_destroy() under c_cm_lock?

>  	/* Prevent shutdown from tearing down the connection
>  	 * while we're executing. */
>  	if (conn) {

[ ... ]

-- 
Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260912035027.27447-1-achender%40kernel.org