[PATCH 6.6.y 4/4] mctp: route: iterate socket tag list in mctp_lookup_prealloc_tag()

[Sasha Levin <sashal@kernel.org>]

From: Hui Peng <benquike@gmail.com>

[ Upstream commit 26cc0e69cce062cd3aa6fae33074684669c35a71 ]

When a socket transmits a packet with MCTP_TAG_PREALLOC set,
mctp_lookup_prealloc_tag() iterates over the per-netns &mns->keys list
and matches netid, req_tag, peer_addr, and manual_alloc, without
checking whether tmp->sk == &msk->sk. This allows any MCTP socket in the
same network namespace to use and consume another socket's preallocated
tag.

Iterate the socket's own tag list (&msk->keys via sklist) instead of the
namespace-wide &mns->keys list in mctp_lookup_prealloc_tag(), ensuring
that only tags allocated by msk are matched.

Tested in QEMU against Linux 7.3.0-rc3 by allocating a manual tag
(0x18) on socket A via SIOCMCTPALLOCTAG for peer EID 9 and sending a
4-byte message with MCTP_TAG_PREALLOC from socket B in the same network
namespace. On the unfixed kernel, sendto(sock_b) using socket A's
preallocated tag succeeds (ret = 4); with this patch applied,
sendto(sock_b) fails with -ENOENT (errno = 2) while sendto(sock_a)
succeeds (ret = 4).

Fixes: 63ed1aab3d40 ("mctp: Add SIOCMCTP{ALLOC,DROP}TAG ioctls for tag control")
Suggested-by: Jeremy Kerr <jk@codeconstruct.com.au>
Cc: stable@vger.kernel.org
Signed-off-by: Hui Peng <benquike@gmail.com>
Link: https://patch.msgid.link/20260921051002.1656692-1-benquike@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/mctp/route.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/mctp/route.c b/net/mctp/route.c
index d947344113e97..4eb7effb6f2cc 100644
--- a/net/mctp/route.c
+++ b/net/mctp/route.c
@@ -758,7 +758,7 @@ static struct mctp_sk_key *mctp_lookup_prealloc_tag(struct mctp_sock *msk,
 
 	spin_lock_irqsave(&mns->keys_lock, flags);
 
-	hlist_for_each_entry(tmp, &mns->keys, hlist) {
+	hlist_for_each_entry(tmp, &msk->keys, sklist) {
 		if (tmp->net != netid)
 			continue;
 
-- 
2.53.0