[PATCH] ipc: mqueue: reject negative queues_max values

[Yingjie Wang]

fs.mqueue.queues_max is documented as the maximum number of POSIX
message queues. Its sysctl entry uses proc_dointvec, but its backing
field, ipc_namespace::mq_queues_max, is unsigned int.

Consequently, writing -1 is accepted and reads back as -1, while the
stored bit pattern is UINT_MAX. The admission check in do_mq_open()
then permits an effectively unbounded number of queues for callers
without CAP_SYS_RESOURCE, rather than enforcing the configured maximum.

Use proc_dointvec_minmax with a zero lower bound. This rejects negative
input while retaining the previously accepted nonnegative signed-int
range, including zero.

The issue was reproduced on 6.12.80 and 6.12.105. With queues_max=1,
an unprivileged workload could create one of three requested queues and
the rest failed with ENOSPC. With queues_max=-1, all three creations
succeeded. After this change, writing -1 fails with EINVAL, and the
finite and zero-value controls retain their prior behavior.

Signed-off-by: Yingjie Wang <1075151112@xxxxxx>
Fixes: bdc8e5f85f9a ("namespaces: mqueue namespace: adapt sysctl")
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: Codex:gpt-5
---
 ipc/mq_sysctl.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/ipc/mq_sysctl.c b/ipc/mq_sysctl.c
index 0dd12e1..7c4f9d0 100644
--- a/ipc/mq_sysctl.c
+++ b/ipc/mq_sysctl.c
@@ -26,7 +26,8 @@ static const struct ctl_table mq_sysctls[] = {
 		.data		= &init_ipc_ns.mq_queues_max,
 		.maxlen		= sizeof(int),
 		.mode		= 0644,
-		.proc_handler	= proc_dointvec,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= SYSCTL_ZERO,
 	},
 	{
 		.procname	= "msg_max",
-- 
2.43.0