cgroup-v1: Require capabilities to set release_agent

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@gmail.com>
Tested-by: Tabitha Sable <tabitha.c.sable@gmail.com>
Fixes: 81a6a5cdd2c5 ("Task Control Groups: automatic userspace notification of idle cgroups")
Cc: stable@vger.kernel.org # v2.6.24+
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
index 41e0837..0e877db 100644 (file)
--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c
@@ -549,6 +549,14 @@ static ssize_t cgroup_release_agent_write(struct kernfs_open_file *of,
 
        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;
@@ -954,6 +962,12 @@ int cgroup1_parse_param(struct fs_context *fc, struct fs_parameter *param)
                /* Specifying two release agents is forbidden */
                if (ctx->release_agent)
                        return invalfc(fc, "release_agent respecified");
+               /*
+                * Release agent gets called with all capabilities,
+                * require capabilities to set release agent.
+                */
+               if ((fc->user_ns != &init_user_ns) || !capable(CAP_SYS_ADMIN))
+                       return invalfc(fc, "Setting release_agent not allowed");
                ctx->release_agent = param->string;
                param->string = NULL;
                break;