Re: [Bug 177821] New: NULL pointer dereference in list_rcu

[Andrew Morton <akpm@linux-foundation.org>]

(switched to email.  Please respond via emailed reply-to-all, not via the
bugzilla web interface).

On Mon, 17 Oct 2016 13:08:17 +0000 bugzilla-daemon@bugzilla.kernel.org wrote:

> https://bugzilla.kernel.org/show_bug.cgi?id=177821
> 
>             Bug ID: 177821
>            Summary: NULL pointer dereference in list_rcu

Fair enough, I suppose.

Please don't submit patches via bugzilla - it is quite
painful.  Documentation/SubmittingPatches explains the
way to do it.

Here's what I put together.  Note that we do not have your
signed-off-by: for this.  Please send it?



From: Alexander Polakov <apolyakov@beget.ru>
Subject: mm/list_lru.c: avoid error-path NULL pointer deref

As described in https://bugzilla.kernel.org/show_bug.cgi?id=177821:

After some analysis it seems to be that the problem is in alloc_super(). 
In case list_lru_init_memcg() fails it goes into destroy_super(), which
calls list_lru_destroy().

And in list_lru_init() we see that in case memcg_init_list_lru() fails,
lru->node is freed, but not set NULL, which then leads list_lru_destroy()
to believe it is initialized and call memcg_destroy_list_lru(). 
memcg_destroy_list_lru() in turn can access lru->node[i].memcg_lrus, which
is NULL.

[akpm@linux-foundation.org: add comment]
Cc: Vladimir Davydov <vdavydov@parallels.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/list_lru.c |    2 ++
 1 file changed, 2 insertions(+)

diff -puN mm/list_lru.c~a mm/list_lru.c
--- a/mm/list_lru.c~a
+++ a/mm/list_lru.c
@@ -554,6 +554,8 @@ int __list_lru_init(struct list_lru *lru
 	err = memcg_init_list_lru(lru, memcg_aware);
 	if (err) {
 		kfree(lru->node);
+		/* Do this so a list_lru_destroy() doesn't crash: */
+		lru->node = NULL;
 		goto out;
 	}
 
_

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