[PATCH 6.1.y 5/5] mptcp: pm: userspace: fix address ID overflow

[Sasha Levin <sashal@kernel.org>]

From: Qing Luo <luoqing@kylinos.cn>

[ Upstream commit f9f0068e8813d8c10d016b030fc3a320d0b6767c ]

When all MPTCP address IDs (1-255) are exhausted in the userspace PM,
find_next_zero_bit() returns MPTCP_PM_MAX_ADDR_ID + 1 (256). This value
overflows when stored in the u8 field e->addr.id, resulting in ID 0
being stored and the entry being incorrectly added to the list.

ID 0 is reserved for the initial connection in MPTCP, so this overflow
can cause address conflicts.

Note: the in-kernel PM already has an 'endpoints == MPTCP_PM_MAX_ADDR_ID'
check in mptcp_pm_nl_append_new_local_addr() that returns -ERANGE before
reaching find_next_zero_bit(), preventing this overflow. So this fix only
addresses the userspace PM path.

Check the find_next_zero_bit() result against MPTCP_PM_MAX_ADDR_ID and
return -ENOSPC if all IDs are truly exhausted. Move the ID allocation
check before the memory allocation so that the error path does not need
to free the allocated entry.

Fixes: 4638de5aefe5 ("mptcp: handle local addrs announced by userspace PMs")
Cc: stable@vger.kernel.org
Signed-off-by: Qing Luo <luoqing@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260908-net-mptcp-misc-fixes-7-3-rc1-v2-8-df1de70348b6@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/mptcp/pm_userspace.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/net/mptcp/pm_userspace.c b/net/mptcp/pm_userspace.c
index 1bc270276db46..a455303d2ed64 100644
--- a/net/mptcp/pm_userspace.c
+++ b/net/mptcp/pm_userspace.c
@@ -67,6 +67,19 @@ static int mptcp_userspace_pm_append_new_local_addr(struct mptcp_sock *msk,
 	}
 
 	if (!addr_match && !id_match) {
+		unsigned int id;
+
+		if (!entry->addr.id && needs_id) {
+			id = find_next_zero_bit(id_bitmap,
+						MPTCP_PM_MAX_ADDR_ID + 1, 1);
+			if (id > MPTCP_PM_MAX_ADDR_ID) {
+				ret = -ENOSPC;
+				goto append_err;
+			}
+		} else {
+			id = entry->addr.id;
+		}
+
 		/* Memory for the entry is allocated from the
 		 * sock option buffer.
 		 */
@@ -76,10 +89,7 @@ static int mptcp_userspace_pm_append_new_local_addr(struct mptcp_sock *msk,
 			goto append_err;
 		}
 
-		if (!e->addr.id && needs_id)
-			e->addr.id = find_next_zero_bit(id_bitmap,
-							MPTCP_PM_MAX_ADDR_ID + 1,
-							1);
+		e->addr.id = id;
 		list_add_tail_rcu(&e->list, &msk->pm.userspace_pm_local_addr_list);
 		msk->pm.local_addr_used++;
 		ret = e->addr.id;
-- 
2.53.0