linux-kernel - Re: fs: out of bounds on stack in iov_iter

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Message-ID: <55FB75D0.7060403@oracle.com>
Date:	Thu, 17 Sep 2015 22:24:16 -0400
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Al Viro <viro@...IV.linux.org.uk>,
	Andrey Ryabinin <ryabinin.a.a@...il.com>, willy@...ux.intel.com
CC:	Chuck Ebbert <cebbert.lkml@...il.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: fs: out of bounds on stack in iov_iter_advance

On 08/19/2015 01:46 AM, Al Viro wrote:
>> or mapping->a_ops->direct_IO() returned more
>> > than 'count'.
> 	Was there DAX involved?  ->direct_IO() in there is blkdev_direct_IO(),
> which takes rather different paths in those cases...
> 

So I've traced this all the way back to dax_io(). I can trigger this with:

diff --git a/fs/dax.c b/fs/dax.c
index 93bf2f9..2cdb8a5 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -178,6 +178,7 @@ static ssize_t dax_io(struct inode *inode, struct iov_iter *iter,
        if (need_wmb)
                wmb_pmem();

+       WARN_ON((pos == start) && (pos - start > iov_iter_count(iter)));
        return (pos == start) ? retval : pos - start;
 }

So it seems that iter gets moved twice here: once in dax_io(), and once again
back at generic_file_read_iter().

I don't see how it ever worked. Am I missing something?


Thanks,
Sasha
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