2015-08-25 02:51:33

by Aleksa Sarai

Subject: [PATCH v2] cgroup: pids: fix invalid get/put usage

Fix incorrect usage of css_get and css_put to put a different css in
pids_{cancel_,}attach() than the one grabbed in pids_can_attach(). This
could lead to quite serious memory leakage (and unsafe operations on the
putted css).

Signed-off-by: Aleksa Sarai <[email protected]>
---
kernel/cgroup_pids.c | 19 ++++---------------
1 file changed, 4 insertions(+), 15 deletions(-)

diff --git a/kernel/cgroup_pids.c b/kernel/cgroup_pids.c
index d75488824ae2..056564f8dd85 100644
--- a/kernel/cgroup_pids.c
+++ b/kernel/cgroup_pids.c
@@ -173,11 +173,11 @@ static int pids_can_attach(struct cgroup_subsys_state *css,
struct pids_cgroup *old_pids;

/*
- * Grab a ref to each task's css. We don't drop the ref until
- * we either fail and hit ->cancel_attach() or succeed and hit
- * ->attach().
+ * We don't need to grab a ref between here and cancel_attach()
+ * because cgroup_migrate_prepare_src() protects the task's css
+ * from being freed before the migration completed or failed.
*/
- old_css = task_get_css(task, pids_cgrp_id);
+ old_css = task_css(task, pids_cgrp_id);
old_pids = css_pids(old_css);

pids_charge(pids, 1);
@@ -202,19 +202,9 @@ static void pids_cancel_attach(struct cgroup_subsys_state *css,

pids_charge(old_pids, 1);
pids_uncharge(pids, 1);
- css_put(old_css);
}
}

-static void pids_attach(struct cgroup_subsys_state *css,
- struct cgroup_taskset *tset)
-{
- struct task_struct *task;
-
- cgroup_taskset_for_each(task, tset)
- css_put(task_css(task, pids_cgrp_id));
-}
-
static int pids_can_fork(struct task_struct *task, void **priv_p)
{
struct cgroup_subsys_state *css;
@@ -354,7 +344,6 @@ static struct cftype pids_files[] = {
struct cgroup_subsys pids_cgrp_subsys = {
.css_alloc = pids_css_alloc,
.css_free = pids_css_free,
- .attach = pids_attach,
.can_attach = pids_can_attach,
.cancel_attach = pids_cancel_attach,
.can_fork = pids_can_fork,
--
2.5.0

2015-08-25 18:20:12

by Tejun Heo

Subject: Re: [PATCH v2] cgroup: pids: fix invalid get/put usage

On Tue, Aug 25, 2015 at 12:50:44PM +1000, Aleksa Sarai wrote:
> Fix incorrect usage of css_get and css_put to put a different css in
> pids_{cancel_,}attach() than the one grabbed in pids_can_attach(). This
> could lead to quite serious memory leakage (and unsafe operations on the
> putted css).
>
> Signed-off-by: Aleksa Sarai <[email protected]>

Applied to cgroup/for-4.3 w/ minor comment update.

Thanks.

--
tejun