[PATCH net v2] mctp: route: iterate socket tag list in mctp_lookup_prealloc_tag()

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
Assisted-by: LLM
Signed-off-by: Hui Peng <redacted>
---
Changes in v2:
- Iterate &msk->keys via sklist instead of walking &mns->keys and checking
  tmp->sk != &msk->sk, as suggested by Jeremy Kerr.

 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 2ce0d9a39bd3..f2b0d8b21f0e 100644
--- a/net/mctp/route.c
+++ b/net/mctp/route.c

@@ -826,7 +826,7 @@ 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;
 

-- 
2.49.0