[PATCH 4.9 1/2] cgroup-v1: Require capabilities to set release_agent

[Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>]

From: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>

commit 24f6008564183aa120d07c03d9289519c2fe02af upstream.

The cgroup release_agent is called with call_usermodehelper.  The function
call_usermodehelper starts the release_agent with a full set fo capabilities.
Therefore require capabilities when setting the release_agaent.

Reported-by: Tabitha Sable <tabitha.c.sable@xxxxxxxxx>
Tested-by: Tabitha Sable <tabitha.c.sable@xxxxxxxxx>
Fixes: 81a6a5cdd2c5 ("Task Control Groups: automatic userspace notification of idle cgroups")
Cc: stable@xxxxxxxxxxxxxxx # v2.6.24+
Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
[mkoutny: Adjust for pre-fs_context, duplicate mount/remount check, drop log messages.]
Acked-by: Michal Koutný <mkoutny@xxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 kernel/cgroup.c |   26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1854,6 +1854,7 @@ static int cgroup_remount(struct kernfs_
 {
 	int ret = 0;
 	struct cgroup_root *root = cgroup_root_from_kf(kf_root);
+	struct cgroup_namespace *ns = current->nsproxy->cgroup_ns;
 	struct cgroup_sb_opts opts;
 	u16 added_mask, removed_mask;
 
@@ -1873,6 +1874,13 @@ static int cgroup_remount(struct kernfs_
 		pr_warn("option changes via remount are deprecated (pid=%d comm=%s)\n",
 			task_tgid_nr(current), current->comm);
 
+	/* See cgroup_mount release_agent handling */
+	if (opts.release_agent &&
+	    ((ns->user_ns != &init_user_ns) || !capable(CAP_SYS_ADMIN))) {
+		ret = -EINVAL;
+		goto out_unlock;
+	}
+
 	added_mask = opts.subsys_mask & ~root->subsys_mask;
 	removed_mask = root->subsys_mask & ~opts.subsys_mask;
 
@@ -2248,6 +2256,16 @@ static struct dentry *cgroup_mount(struc
 		goto out_unlock;
 	}
 
+	/*
+	 * Release agent gets called with all capabilities,
+	 * require capabilities to set release agent.
+	 */
+	if (opts.release_agent &&
+	    ((ns->user_ns != &init_user_ns) || !capable(CAP_SYS_ADMIN))) {
+		ret = -EINVAL;
+		goto out_unlock;
+	}
+
 	root = kzalloc(sizeof(*root), GFP_KERNEL);
 	if (!root) {
 		ret = -ENOMEM;
@@ -3026,6 +3044,14 @@ static ssize_t cgroup_release_agent_writ
 
 	BUILD_BUG_ON(sizeof(cgrp->root->release_agent_path) < PATH_MAX);
 
+	/*
+	 * 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))
+		return -EPERM;
+
 	cgrp = cgroup_kn_lock_live(of->kn, false);
 	if (!cgrp)
 		return -ENODEV;