linux-kernel - Re: [patch] xfs: check for underflow in xfs_iformat

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Message-ID: <20130823173613.GT5262@sgi.com>
Date:	Fri, 23 Aug 2013 12:36:13 -0500
From:	Ben Myers <bpm@....com>
To:	Dan Carpenter <dan.carpenter@...cle.com>
Cc:	Alex Elder <elder@...nel.org>, kernel-janitors@...r.kernel.org,
	linux-kernel@...r.kernel.org, xfs@....sgi.com,
	Jeff Liu <jeff.liu@...cle.com>,
	Dave Chinner <david@...morbit.com>
Subject: Re: [patch] xfs: check for underflow in xfs_iformat_fork()

Dan,

On Fri, Aug 16, 2013 at 08:26:50AM +1000, Dave Chinner wrote:
> On Thu, Aug 15, 2013 at 09:37:06AM -0500, Ben Myers wrote:
> > Hey Dan & Jeff,
> > 
> > On Thu, Aug 15, 2013 at 06:10:43PM +0800, Jeff Liu wrote:
> > > On 08/15/2013 01:53 PM, Dan Carpenter wrote:
> > > 
> > > > The "di_size" variable comes from the disk and it's a signed 64 bit.
> > > > We check the upper limit but we should check for negative numbers as
> > > > well.
> > > > 
> > > > Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>
> > > > 
> > > > diff --git a/fs/xfs/xfs_inode_fork.c b/fs/xfs/xfs_inode_fork.c
> > > > index 123971b..849fc70 100644
> > > > --- a/fs/xfs/xfs_inode_fork.c
> > > > +++ b/fs/xfs/xfs_inode_fork.c
> > > > @@ -167,7 +167,8 @@ xfs_iformat_fork(
> > > >  			}
> > > >  
> > > >  			di_size = be64_to_cpu(dip->di_size);
> > > > -			if (unlikely(di_size > XFS_DFORK_DSIZE(dip, ip->i_mount))) {
> > > > +			if (unlikely(di_size < 0 ||
> > > 
> > > But the di_size is initialized to ZERO while allocating a new inode on disk.
> > > I wonder if that is better to ASSERT in this case because the current check
> > > is used to make sure that the item is inlined, or we don't need it at all.
> > 
> > Hmm.  Dan's additional check looks good to me.  In this case I'd say the forced
> > shutdown is more appropriate than an assert, because here we're reading the
> > inode from disk, as opposed to looking at a structure that is already incore
> > which we think we've initialized.  We want to handle unexpected inputs from
> > disk without crashing even if we are CONFIG_XFS_DEBUG.
> 
> There are lots of places where we only check di_size to be greater
> than some value, and don't check for it being less than zero. Hence
> I think that a better solution might be to di_size unsigned as that
> will catch "negative" sizes for all types of situations.

What do you say to making di_size unsigned?  Any interest?

Thanks,
Ben
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