[PATCH 3.2 131/149] cgroup: fail if monitored file andevent_control are in different cgroup

[Ben Hutchings]

3.2.52-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Li Zefan <lizefan@xxxxxxxxxx>

commit f169007b2773f285e098cb84c74aac0154d65ff7 upstream.

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@xxxxxxxxxx>
Acked-by: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 kernel/cgroup.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -3504,6 +3504,7 @@ static int cgroup_write_event_control(st
 				      const char *buffer)
 {
 	struct cgroup_event *event = NULL;
+	struct cgroup *cgrp_cfile;
 	unsigned int efd, cfd;
 	struct file *efile = NULL;
 	struct file *cfile = NULL;
@@ -3559,6 +3560,16 @@ static int cgroup_write_event_control(st
 		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;

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