cgroup: fail if monitored file and event_control are in different cgroup

If we pass fd of memory.usage_in_bytes of cgroup A to cgroup.event_control
of cgroup B, then we won't get memory usage notification from A but B!

What's worse, if A and B are in different mount hierarchy, we'll end up
accessing NULL pointer!

Disallow this kind of invalid usage.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Acked-by: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 11 insertions, 0 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index feda81487be4..b5c64327e712 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -3856,6 +3856,7 @@ static int cgroup_write_event_control(struct cgroup *cgrp, struct cftype *cft,
                                       const char *buffer)
 {
         struct cgroup_event *event = NULL;
+        struct cgroup *cgrp_cfile;
         unsigned int efd, cfd;
         struct file *efile = NULL;
         struct file *cfile = NULL;
@@ -3911,6 +3912,16 @@ static int cgroup_write_event_control(struct cgroup *cgrp, struct cftype *cft,
                 goto fail;
         }
 
+        /*
+         * The file to be monitored must be in the same cgroup as
+         * cgroup.event_control is.
+         */
+        cgrp_cfile = __d_cgrp(cfile->f_dentry->d_parent);
+        if (cgrp_cfile != cgrp) {
+                ret = -EINVAL;
+                goto fail;
+        }
+
         if (!event->cft->register_event || !event->cft->unregister_event) {
                 ret = -EINVAL;
                 goto fail;