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

[Alexander Polakov <apolyakov@beget.ru>]

> On 18 Oct 2016, at 03:10, Andrew Morton <akpm@linux-foundation.org> wrote:
> 
> 
> (resend due to "vdavydov@virtuozzo.com Unrouteable address")
> 
> (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?

Sorry for the bugzilla thing, here's the patch with Signed-off-by added.
Hope I did it right.

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: Alexander Polakov <apolyakov@beget.ru>
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;
	}

_


> 
> 
> 
> 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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