[PATCH net] net: mctp: free reassembly head skb when key_add fails on SOM

[Joas Antonio dos Santos <joasantonio108@gmail.com>]

When a start-of-message fragment creates a new reassembly key on the
broadcast/bind path, mctp_frag_queue() stores the skb as
key->reasm_head before the key is made observable with mctp_key_add().

If mctp_key_add() fails (a race with another SOM packet for the same
source/dest/tag, or the receiving socket going away), the error path
only drops the key reference via mctp_key_unref().  The final key
destructor frees the key itself but not key->reasm_head, and since the
key was never added to any list, neither the expiry worker nor socket
teardown can reach it to release the queued fragment.  The skb is
leaked.

Free key->reasm_head on the key_add failure path before releasing the
key.  Other teardown paths (__mctp_key_done_in(), EOM delivery) already
clear reasm_head before the final unref, so this only affects the
failure path.

Fixes: 4a992bbd3650 ("mctp: Implement message fragmentation & reassembly")
Signed-off-by: Joas Antonio dos Santos <joasantonio108@gmail.com>
Assisted-by: Yaga Agent HackerSec Claude:claude-opus-5-5
---
Found by code review.  The reassembly/key lifetime was traced in
net/mctp/route.c: mctp_key_unref()'s destructor frees the key but not
key->reasm_head, and only __mctp_key_done_in() and the EOM delivery
path clear reasm_head, neither of which runs on the mctp_key_add()
failure path.  Not yet build- or runtime-tested (build environment
temporarily unavailable); the change is a one-line kfree_skb() on an
error path.

 net/mctp/route.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/net/mctp/route.c b/net/mctp/route.c
index e7c95eeac..3d79c2031 100644
--- a/net/mctp/route.c
+++ b/net/mctp/route.c
@@ -552,8 +552,16 @@ static int mctp_dst_input(struct mctp_dst *dst, struct sk_buff *skb)
 			 * can do is drop.
 			 */
 			rc = mctp_key_add(key, msk);
-			if (!rc)
+			if (!rc) {
 				trace_mctp_key_acquire(key);
+			} else {
+				/* key_add failed: the key is not observable by
+				 * any cleanup path, so free the fragment we
+				 * just queued on it to avoid leaking the skb.
+				 */
+				kfree_skb(key->reasm_head);
+				key->reasm_head = NULL;
+			}
 
 			/* we don't need to release key->lock on exit, so
 			 * clean up here and suppress the unlock via