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net/rds: include the scope id in the sendmsg connection cache check

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rds_sendmsg() reuses the connection cached in rs->rs_conn when its
peer address and ToS match the request.  The interface index is part
of a connection's identity as well: rds_conn_create_outgoing() passes
the request's scope_id down as dev_if, and rds_conn_lookup() compares
c_dev_if, so sends to the same link-local address through two
interfaces are two different connections.  The cache-hit test never
looked at it.

A socket bound to a non-link-local address has rs_bound_scope_id 0,
and the scope check at the top of rds_sendmsg() accepts any non-zero
destination scope for such a socket.  So after a send to fe80::x%ifA,
a send to fe80::x%ifB hits the cached ifA connection and the datagram
leaves through ifA, to whichever peer answers to that address there.

Compare c_dev_if with the request's scope_id in the cache test, so
that such a send takes the lookup path and finds, or creates, the ifB
connection - the same key the lookup uses.  A send without a
destination takes its scope from the socket, and the connection it
created was keyed on that same scope, so the comparison holds for it
too; the next patch gives that send the connected peer's own scope.

Found by inspection while reworking this cache for the connection
lifetime series on net-next; not observed in the field.

Fixes: 1e2b44e78e ("rds: Enable RDS IPv6 support")
Signed-off-by: Allison Henderson <achender@kernel.org>
Link: https://patch.msgid.link/20261020002816.3087414-2-achender@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Allison Henderson 2026-10-19 17:28:15 -07:00 committed by Jakub Kicinski
parent 520631554a
commit b7dfff5e3e
1 changed files with 2 additions and 1 deletions
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3
net/rds/send.c
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@ -1342,7 +1342,8 @@ int rds_sendmsg(struct socket *sock, struct msghdr *msg, size_t payload_len)
/* rds_conn_create has a spinlock that runs with IRQ off.
* Caching the conn in the socket helps a lot. */
if (rs->rs_conn && ipv6_addr_equal(&rs->rs_conn->c_faddr, &daddr) &&
rs->rs_tos == rs->rs_conn->c_tos) {
rs->rs_tos == rs->rs_conn->c_tos &&
rs->rs_conn->c_dev_if == scope_id) {
conn = rs->rs_conn;
} else {
conn = rds_conn_create_outgoing(sock_net(sock->sk),