linux-kernel - Re: [PATCH] vfs: Fix use-after-free of vfsmount by mnt_drop

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Date:	Tue, 23 Feb 2010 06:13:59 +0900
From:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To:	Valerie Aurora <vaurora@...hat.com>
Cc:	Al Viro <viro@...iv.linux.org.uk>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] vfs: Fix use-after-free of vfsmount by mnt_drop_write()

Valerie Aurora <vaurora@...hat.com> writes:

>> Those might be assuming that the nd->path.mnt refcnt is keeped by
>> filp after nameidata_to_filp().
>> 
>> It's wrong if nameidata_to_filp() returned the error.  (nd->path and
>> filp are invalid)
>> 
>> Instead to use deep knowledge of nameidata_to_filp() internal, this
>> thinks the nd->path is invalid after nameidata_to_filp(). So, this
>> just take refcnt for mnt_want/drop_write().
>
> You might take a look at the patch I just posted:
>
> Subject: [RFC PATCH] VFS: Simplify truncate logic in do_filp_open()
>
> If that patch is correct, it will considerably simplify the second
> part of your patch.

[...]
>>  	/*
>> -	 * It is now safe to drop the mnt write
>> -	 * because the filp has had a write taken
>> -	 * on its behalf.
>> +	 * It is now safe to drop the mnt write because the filp has
>> +	 * had a write taken on its behalf. (NOTE: since O_TRUNC can
>> +	 * be used with O_RDONLY, this needs to cover truncate path)
>>  	 */
>> -	if (will_truncate)
>> -		mnt_drop_write(nd.path.mnt);
>> +	if (will_truncate) {
>> +		mnt_drop_write(mnt_writer);
>> +		mntput(mnt_writer);
>> +	}

Unfortunately, no. I already noted that we can't it simply. ;)

Thanks.
-- 
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
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