linux-kernel - Re: Bad ext3/nfs DoS bug

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Message-ID: <20060720160639.GF25111@atrey.karlin.mff.cuni.cz>
Date:	Thu, 20 Jul 2006 18:06:39 +0200
From:	Jan Kara <jack@...e.cz>
To:	Neil Brown <neilb@...e.de>
Cc:	James <20@...ingley.org>, Marcel Holtmann <marcel@...tmann.org>,
	linux-kernel@...r.kernel.org, akpm@...l.org, sct@...hat.com
Subject: Re: Bad ext3/nfs DoS bug

> On Wednesday July 19, jack@...e.cz wrote:
> > > So what happens next? Is the ext3 maintainer on sabatical,
> > > or am I expected to submit a patch to fix this?
> >   I guess people are mostly busy with OLS and such so maybe they missed
> > the discussion.. Giving CC to relevant people to catch their attention
> > :)
> >   Andrew, Stephen: James has come across a nasty bug (potentially remote
> > DoS). NFS extracts inode number from a filehandle and the inode number
> > eventually ends in ext3_read_inode(). Now if the inode number is bogus,
> > ext3_get_inode_block() calls ext3_error() and filesystem is remounted
> > RO or whatever else is configured. That is quite undesirable in this
> > case.
> >   Now the easy "fix" is to change ext3_error() to ext3_warning() but an
> > attacker flooding your logs with warnings is probably not good either
> > and in case the inode number comes from ext3 itself we should really do
> > ext3_error() as there is some corruption in the fs.
> >   Better fix would be to add a flag to read_inode() saying that the inode
> > number is from untrusted source (but that means changing a prototype of a
> > function every fs uses) and change export_iget() to pass this flag. Yet
> > another solution would be to make ext3 implement its own get_dentry() export
> > function and pass the flag internally...
> >   What do you think is the best solution?
> 
> I think that a good solution (hard to say if it is the best) is to
> remove that error message altogether, and put it where inode numbers
> are read out of directories.  Something like the following patch -
> compile tested only.
  Yes, that looks fine too. I did not realize that we get the inode
number only in a few places. Maybe we could wrap the checks in a
function (possibly inline) so that the checks are just in one place?

								Honza
-- 
Jan Kara <jack@...e.cz>
SuSE CR Labs
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