cgroups: fix invalid cgrp->dentry before cgroup has been completely removed

commit 24eb089950ce44603b30a3145a2c8520e2b55bb1 upstream

This fixes an oops when reading /proc/sched_debug.

A cgroup won't be removed completely until finishing cgroup_diput(), so we
shouldn't invalidate cgrp->dentry in cgroup_rmdir().  Otherwise, when a
group is being removed while cgroup_path() gets called, we may trigger
NULL dereference BUG.

The bug can be reproduced:

 # cat test.sh
 #!/bin/sh
 mount -t cgroup -o cpu xxx /mnt
 for (( ; ; ))
 {
	mkdir /mnt/sub
	rmdir /mnt/sub
 }
 # ./test.sh &
 # cat /proc/sched_debug

BUG: unable to handle kernel NULL pointer dereference at 00000038
IP: [<c045a47f>] cgroup_path+0x39/0x90
..
Call Trace:
 [<c0420344>] ? print_cfs_rq+0x6e/0x75d
 [<c0421160>] ? sched_debug_show+0x72d/0xc1e
..

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Paul Menage <menage@google.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index a0123d75ec9a7624f897bc2984ca4009b61abfbc..d68bf2bc5caeadc6fb975fe7de4b22583c1c4355 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2443,7 +2443,6 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
        list_del(&cgrp->sibling);
        spin_lock(&cgrp->dentry->d_lock);
        d = dget(cgrp->dentry);
-       cgrp->dentry = NULL;
        spin_unlock(&d->d_lock);
 
        cgroup_d_remove_dir(d);