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Date: Wed, 9 Jul 2014 13:31:38 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Sasha Levin <sasha.levin@oracle.com>, David Rientjes <rientjes@google.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>, Jan Kara <jack@suse.cz>,
        Dave Jones <davej@redhat.com>, LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: fs: use after free in /proc/pid/mountinfo
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On Wed, 9 Jul 2014 16:24:06 +0200 Heiko Carstens <heiko.carstens@de.ibm.com> wrote:

> 058504edd026 ("fs/seq_file: fallback to vmalloc allocation") is suspected to
> cause a crash. Actually I can't reprocude the crash nor would I be able to
> tell how the commit could cause the crash.
> Anyway, I'll be offline for the next 2.5 weeks. So if Sasha could confirm
> that reverting the patch actually does fix the crash, please revert the
> commit, unless somebody else can make sense of the report of course.
> 
> I'm still wondering how Sasha could reproduce the crash.

This really sucks :(

I suppose I'll queue up a revert of 058504edd026 as a for-3.16 bandaid.

Sasha, it would be great if you could delve a bit further into this,
see if we can identify a way for others to reproduce the bug.

Did you test that little patchlet which Heiko sent?

--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -34,12 +34,7 @@ static void seq_set_overflow(struct seq_file *m)
 
 static void *seq_buf_alloc(unsigned long size)
 {
-	void *buf;
-
-	buf = kmalloc(size, GFP_KERNEL | __GFP_NOWARN);
-	if (!buf && size > PAGE_SIZE)
-		buf = vmalloc(size);
-	return buf;
+	return kmalloc(size, GFP_KERNEL);
 }
 
 /**


It would also be interesting to try the opposite: always use vmalloc().
That may make it easier to trigger the bug.