2013-04-26 02:54:25

by Zefan Li

Subject: [PATCH] cgroup: fix use-after-free when umounting cgroupfs

Try:
# mount -t cgroup xxx /cgroup
# mkdir /cgroup/sub && rmdir /cgroup/sub && umount /cgroup

And you might see this:

ida_remove called for id=1 which is not allocated.

It's because cgroup_kill_sb() is called to destroy root->cgroup_ida
and free cgrp->root before ida_simple_removed() is called. What's
worse is we're accessing cgrp->root while it has been freed.

Signed-off-by: Li Zefan <[email protected]>
---
kernel/cgroup.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 6780459..a45aa12 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -845,9 +845,12 @@ static void cgroup_free_fn(struct work_struct *work)
*/
dput(cgrp->parent->dentry);

+ ida_simple_remove(&cgrp->root->cgroup_ida, cgrp->id);
+
/*
* Drop the active superblock reference that we took when we
- * created the cgroup
+ * created the cgroup. This will free cgrp->root, if we are
+ * holding the last reference to @sb.
*/
deactivate_super(cgrp->root->sb);

@@ -859,7 +862,6 @@ static void cgroup_free_fn(struct work_struct *work)

simple_xattrs_free(&cgrp->xattrs);

- ida_simple_remove(&cgrp->root->cgroup_ida, cgrp->id);
kfree(rcu_dereference_raw(cgrp->name));
kfree(cgrp);
}
--
1.8.0.2

2013-04-26 18:59:30

by Tejun Heo

Subject: Re: [PATCH] cgroup: fix use-after-free when umounting cgroupfs

On Fri, Apr 26, 2013 at 10:54:11AM +0800, Li Zefan wrote:
> Try:
> # mount -t cgroup xxx /cgroup
> # mkdir /cgroup/sub && rmdir /cgroup/sub && umount /cgroup
>
> And you might see this:
>
> ida_remove called for id=1 which is not allocated.
>
> It's because cgroup_kill_sb() is called to destroy root->cgroup_ida
> and free cgrp->root before ida_simple_removed() is called. What's
> worse is we're accessing cgrp->root while it has been freed.
>
> Signed-off-by: Li Zefan <[email protected]>

Applied to cgroup/for-3.10.

Thanks.

--
tejun