linux-ext4 - Re: [PATCH 2/2] ext2fs: Fix off-by-one in dx_grow

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Date:   Tue, 31 Mar 2020 13:33:03 +0200
From:   Lukas Czerner <lczerner@...hat.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Ted Tso <tytso@....edu>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 2/2] ext2fs: Fix off-by-one in dx_grow_tree()

On Mon, Mar 30, 2020 at 04:55:31PM +0200, Jan Kara wrote:
> On Mon 30-03-20 15:27:12, Lukas Czerner wrote:
> > On Mon, Mar 30, 2020 at 11:09:32AM +0200, Jan Kara wrote:
> > > There is an off-by-one error in dx_grow_tree() when checking whether we
> > > can add another level to the tree. Thus we can grow tree too much
> > > leading to possible crashes in the library or corrupted filesystem. Fix
> > > the bug.
> > 
> > Looks good, thanks
> > 
> > Reviewed-by: Lukas Czerner <lczerner@...hat.com>
> > 
> > Don't we have basically the same off-by-one in
> > e2fsck/pass1.c handle_htree() ?
> > 
> >        if ((root->indirect_levels > ext2_dir_htree_level(fs)) &&
> >            fix_problem(ctx, PR_1_HTREE_DEPTH, pctx))
> 
> I don't think so. It is indeed correct for root->indirect_levels to be
> equal to ext2_dir_htree_level(). However dx_grow_tree() is going to
> increase root->indirect_levels by 1 which is where tree would become
> invalid...
> 
> 								Honza

Hmm, are you sure ? I think the names are very confusing

root->indirect_levels is zero based, while ext2_dir_htree_level()
returns the maximum number of levels (that is 3 by default). If I am
right then indirect_levels must always be smaller then
ext2_dir_htree_level() and that is how we use it everywhere else - the
palce I am pointing out is an exception and I think it's a bug.

Indeed it looks like the bug got introduced in
3f0cf647539970474be8f607017ca7eccfc2fbbe

-       if ((root->indirect_levels > 1) &&
+       if ((root->indirect_levels > ext2_dir_htree_level(fs)) &&

Or am I missing something ?

-Lukas

> 
> > > 
> > > Signed-off-by: Jan Kara <jack@...e.cz>
> > > ---
> > >  lib/ext2fs/link.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/lib/ext2fs/link.c b/lib/ext2fs/link.c
> > > index 7b5bb022117c..469eea8cd06d 100644
> > > --- a/lib/ext2fs/link.c
> > > +++ b/lib/ext2fs/link.c
> > > @@ -473,7 +473,7 @@ static errcode_t dx_grow_tree(ext2_filsys fs, ext2_ino_t dir,
> > >  		    ext2fs_le16_to_cpu(info->frames[i].head->limit))
> > >  			break;
> > >  	/* Need to grow tree depth? */
> > > -	if (i < 0 && info->levels > ext2_dir_htree_level(fs))
> > > +	if (i < 0 && info->levels >= ext2_dir_htree_level(fs))
> > >  		return EXT2_ET_DIR_NO_SPACE;
> > >  	lblk = size / fs->blocksize;
> > >  	size += fs->blocksize;
> > > -- 
> > > 2.16.4
> > > 
> > 
> -- 
> Jan Kara <jack@...e.com>
> SUSE Labs, CR
>