[PATCH v2] xfs: fix possible overflow in xfs_ioc_trim()

[Lukas Czerner]

On Tue, 6 Sep 2011, Christoph Hellwig wrote:

> On Tue, Sep 06, 2011 at 05:29:37PM +0200, Lukas Czerner wrote:
> > In xfs_ioc_trim it is possible that start+len might overflow. Fix it by
> > decrementing the len so that start+len equals to the file system size in
> > the worst case.
> > 
> > Signed-off-by: Lukas Czerner <lczerner at redhat.com>
> 
> > @@ -146,6 +146,7 @@ xfs_ioc_trim(
> >  	unsigned int		granularity = q->limits.discard_granularity;
> >  	struct fstrim_range	range;
> >  	xfs_fsblock_t		start, len, minlen;
> > +	xfs_fsblock_t		max_blks = mp->m_sb.sb_dblocks;
> >  	xfs_agnumber_t		start_agno, end_agno, agno;
> >  	__uint64_t		blocks_trimmed = 0;
> >  	int			error, last_error = 0;
> > @@ -171,7 +172,8 @@ xfs_ioc_trim(
> >  	start_agno = XFS_FSB_TO_AGNO(mp, start);
> >  	if (start_agno >= mp->m_sb.sb_agcount)
> >  		return -XFS_ERROR(EINVAL);
> > -
> > +	if (len > max_blks)
> > +		len = max_blks - start;
> 
> Is this really the correct check?
> 
> Shouldn't it be
> 
> 	if (start + len > max_blks)
> 		len = max_blks - start;
> 
> I'd also just use the mp->m_sb.sb_dblocks value directly instead
> of assigning it to a local variable.
> 

Agh, you're right. I am bit too hasty I guess. I thought that

if (start_agno >= mp->m_sb.sb_agcount)
	return -XFS_ERROR(EINVAL);

will cover us from the unreasonably big start, however if the file
system has really huge number of AGs than it will fail to prevent the
overflow, I am not sure if that is possible to happen, but what you
proposed is definitely better.

Thanks!
-Lukas