cgroup-v1: Correct privileges check in release_agent writes

The idea is to check: a) the owning user_ns of cgroup_ns, b)
capabilities in init_user_ns.

The commit 24f60085 ("cgroup-v1: Require capabilities to set
release_agent") got this wrong in the write handler of release_agent
since it checked user_ns of the opener (may be different from the owning
user_ns of cgroup_ns).
Secondly, to avoid possibly confused deputy, the capability of the
opener must be checked.

Fixes: 24f60085 ("cgroup-v1: Require capabilities to set release_agent")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/stable/20220216121142.GB30035@blackbody.suse.cz/Signed-off-by: Michal Koutný <mkoutny@suse.com>
Reviewed-by: Masami Ichikawa(CIP) <masami.ichikawa@cybertrust.co.jp>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup/cgroup-v1.c

@@ -546,6 +546,7 @@ static ssize_t cgroup_release_agent_write(struct kernfs_open_file *of,
 					  char *buf, size_t nbytes, loff_t off)
 {
 	struct cgroup *cgrp;
+	struct cgroup_file_ctx *ctx;
 
 	BUILD_BUG_ON(sizeof(cgrp->root->release_agent_path) < PATH_MAX);
 
@@ -553,8 +554,9 @@ static ssize_t cgroup_release_agent_write(struct kernfs_open_file *of,
 	 * Release agent gets called with all capabilities,
 	 * require capabilities to set release agent.
 	 */
-	if ((of->file->f_cred->user_ns != &init_user_ns) ||
-	    !capable(CAP_SYS_ADMIN))
+	ctx = of->priv;
+	if ((ctx->ns->user_ns != &init_user_ns) ||
+	    !file_ns_capable(of->file, &init_user_ns, CAP_SYS_ADMIN))
 		return -EPERM;
 
 	cgrp = cgroup_kn_lock_live(of->kn, false);