Re: [PATCH] Btrfs: memset to avoid stale content in btree node block

[David Sterba <dsterba@suse.cz>]

On Tue, Sep 20, 2016 at 10:57:41AM -0700, Liu Bo wrote:
> On Tue, Sep 20, 2016 at 03:16:36PM +0200, David Sterba wrote:
> > On Wed, Sep 14, 2016 at 05:22:57PM -0700, Liu Bo wrote:
> > > During updating btree, we could push items between sibling
> > > nodes/leaves, for leaves data sections starts reversely from
> > > the end of the block while for nodes we only have key pairs
> > > which are stored one by one from the start of the block.
> > > 
> > > So we could do try to push key pairs from one node to the next
> > > node right in the tree, and after that, we update the node's
> > > nritems to reflect the correct end while leaving the stale
> > > content in the node.  One may intentionally corrupt the fs
> > > image and access the stale content by bumping the nritems and
> > > causes various crashes.
> > > 
> > > This takes the in-memory @nritems as the correct one and
> > > gets to memset the unused part of a btree node.
> > > 
> > > Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
> > 
> > Reviewed-by: David Sterba <dsterba@suse.com>
> > 
> > > ---
> > >  fs/btrfs/extent_io.c | 11 +++++++++++
> > >  1 file changed, 11 insertions(+)
> > > 
> > > diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> > > index c2325c3..56c9dee 100644
> > > --- a/fs/btrfs/extent_io.c
> > > +++ b/fs/btrfs/extent_io.c
> > > @@ -3732,6 +3732,17 @@ static noinline_for_stack int write_one_eb(struct extent_buffer *eb,
> > >  	if (btrfs_header_owner(eb) == BTRFS_TREE_LOG_OBJECTID)
> > >  		bio_flags = EXTENT_BIO_TREE_LOG;
> > >  
> > > +	/* set btree node beyond nritems with 0 to avoid stale content */
> > > +	if (btrfs_header_level(eb) > 0) {
> > 
> > We can do the same for leaves.
> 
> In theory, the problem also applies for leaves, but I haven't got a
> reproducer for leaf case.
> 
> So I'll update a v2 with leaf memset, please review that part more
> carefully :)

You can keep it a separate patch, this one is fine. I didn't expect to
reproduce a crash with a bogus nritems in a leaf but rather apply the
same on a leaf buffer. The magic formula is (please verify)

start = nr * sizeof(struct btrfs_disk_key);
end = nr ? btrfs_item_offset(eb, btrfs_item_nr(nr - 1)) : eb->len;