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Subject[PATCH] 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>
---
kernel/cgroup.c | 11 +++++++++++
1 file changed, 11 insertions(+)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 65c8101..a41673c 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -3911,6 +3911,7 @@ 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;
@@ -3966,6 +3967,16 @@ 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;
--
1.8.0.2

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