linux-ext4 - Re: [PATCH] fs/ext{3,4}: fix potential race when setversion ioctl updates inode

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Date:	Thu, 5 Jan 2012 01:40:09 +0100
From:	Djalal Harouni <tixxdz@...ndz.org>
To:	Jan Kara <jack@...e.cz>
Cc:	Andreas Dilger <adilger.kernel@...ger.ca>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Darrick J. Wong" <djwong@...ibm.com>,
	Theodore Ts'o <tytso@....edu>,
	Yongqiang Yang <xiaoqiangnk@...il.com>,
	ext4 development <linux-ext4@...r.kernel.org>,
	linux-kernel Mailing List <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] fs/ext{3,4}: fix potential race when setversion ioctl
 updates inode

On Thu, Jan 05, 2012 at 12:32:54AM +0100, Jan Kara wrote:
> On Wed 04-01-12 16:15:04, Andreas Dilger wrote:
> > On 2012-01-04, at 10:46 AM, Jan Kara wrote:
> > > On Tue 03-01-12 02:31:52, Djalal Harouni wrote:
> > >> 
> > >> The EXT{3,4}_IOC_SETVERSION ioctl() updates the inode without i_mutex,
> > >> this can lead to a race with the other operations that update the same
> > >> inode.
> > >> 
> > >> Patch tested.
> > > 
> > >  OK, so I've taken the patch into my tree, just updated the changelog
> > > which result of our discussion in this thread. I also took the ext4 part
> > > since there is no risk of conflict and the patch looks obvious.
> > 
> > Actually, I'd like to hear more about whether this is a real problem, or
> > if it is just a theoretical problem found during code inspection or from
> > some static code analysis tool?
>   As far as I understood that was just a theoretical issue and I applied
> the patch just on the grounds that it is more consistent to use i_mutex for
> i_generation changes.
This was found using a static code analysis tool (currently a PoC) which
is a part of a research project at our university.

And later, code inspection confirms that i_ctime updates can be disturbed.

I should have specified this. Sorry.

> > With the metadata checksum feature we were discussing using the inode
> > generation as part of the seed for the directory leaf block checksum, so
> > that it wasn't possible to incorrectly access stale directory blocks from
> > a previous incarnation of the same inode number.
> > 
> > We were discussing just disabling this ioctl on filesystems with metadata
> > checksums, and printing a deprecation warning for filesystems without that
> > feature enabled.  I'm not aware of any real-world use for this ioctl, since
> > NFS cannot use it to reconstruct handles because there's no API to allocate
> > an inode with a specific number, so setting the generation is pointless.
>   OK, I didn't know this. I'm fine with deprecating the ioctl if it's
> useless but since that's going to take a while I think the cleanup still
> makes some sense.
Actually I've grepped this ioctl but did not found use cases, but as
ext{3,2} also support it, I did not say anything (this is old, there is
even the EXT4_IOC_SETVERSION_OLD ioctl ?). I don't know if this ioctl is
used or not.

Only the reiserfs and ext{2,3,4} filesystems support this ioctl. The reiserfs
do not use mutexes at all, even in the REISERFS_IOC_SETFLAGS ioctl which will
test and set _all_ the possible values of the i_flags field.
Perhaps I should also send a patch for this ?

And perhaps ext2 should also be updated.

> 								Honza
> -- 
> Jan Kara <jack@...e.cz>
> SUSE Labs, CR
Thanks for the feedback.

-- 
tixxdz
http://opendz.org
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