Even privileged services should not necessarily be able to see other
privileged service's namespaces so they can't leak information to each
other. Use may_see_all_namespaces() helper that centralizes this policy
until the nstree adapts.

Fixes: a1d220d9dafa ("nsfs: iterate through mount namespaces")
Cc: stable@kernel.org # v6.12+
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/nsfs.c                 | 13 +++++++++++++
 include/linux/ns_common.h |  2 ++
 kernel/nscommon.c         |  6 ++++++
 3 files changed, 21 insertions(+)

diff --git a/fs/nsfs.c b/fs/nsfs.c
index db91de208645..be36c10c38cf 100644
--- a/fs/nsfs.c
+++ b/fs/nsfs.c
@@ -199,6 +199,17 @@ static bool nsfs_ioctl_valid(unsigned int cmd)
 	return false;
 }
 
+static bool may_use_nsfs_ioctl(unsigned int cmd)
+{
+	switch (_IOC_NR(cmd)) {
+	case _IOC_NR(NS_MNT_GET_NEXT):
+		fallthrough;
+	case _IOC_NR(NS_MNT_GET_PREV):
+		return may_see_all_namespaces();
+	}
+	return true;
+}
+
 static long ns_ioctl(struct file *filp, unsigned int ioctl,
 			unsigned long arg)
 {
@@ -214,6 +225,8 @@ static long ns_ioctl(struct file *filp, unsigned int ioctl,
 
 	if (!nsfs_ioctl_valid(ioctl))
 		return -ENOIOCTLCMD;
+	if (!may_use_nsfs_ioctl(ioctl))
+		return -EPERM;
 
 	ns = get_proc_ns(file_inode(filp));
 	switch (ioctl) {
diff --git a/include/linux/ns_common.h b/include/linux/ns_common.h
index 825f5865bfc5..c8e227a3f9e2 100644
--- a/include/linux/ns_common.h
+++ b/include/linux/ns_common.h
@@ -55,6 +55,8 @@ static __always_inline bool is_ns_init_id(const struct ns_common *ns)
 
 #define ns_common_free(__ns) __ns_common_free(to_ns_common((__ns)))
 
+bool may_see_all_namespaces(void);
+
 static __always_inline __must_check int __ns_ref_active_read(const struct ns_common *ns)
 {
 	return atomic_read(&ns->__ns_ref_active);
diff --git a/kernel/nscommon.c b/kernel/nscommon.c
index bdc3c86231d3..3166c1fd844a 100644
--- a/kernel/nscommon.c
+++ b/kernel/nscommon.c
@@ -309,3 +309,9 @@ void __ns_ref_active_get(struct ns_common *ns)
 			return;
 	}
 }
+
+bool may_see_all_namespaces(void)
+{
+	return (task_active_pid_ns(current) == &init_pid_ns) &&
+	       ns_capable_noaudit(init_pid_ns.user_ns, CAP_SYS_ADMIN);
+}

-- 
2.47.3