Subject: Re: [PATCH 47/64] btrfs: Use memset_after() to clear end of struct

From: Kees Cook <[email-masked]>
Date: Wed, 28 Jul 2021 14:56:31 -0700

On Wed, Jul 28, 2021 at 11:42:15AM +0200, David Sterba wrote:
> On Tue, Jul 27, 2021 at 01:58:38PM -0700, Kees Cook wrote:
> > In preparation for FORTIFY_SOURCE performing compile-time and run-time
> > field bounds checking for memset(), avoid intentionally writing across
> > neighboring fields.
> > 
> > Use memset_after() so memset() doesn't get confused about writing
> > beyond the destination member that is intended to be the starting point
> > of zeroing through the end of the struct.
> > 
> > Signed-off-by: Kees Cook <[email-masked]>
> > ---
> >  fs/btrfs/root-tree.c | 5 +----
> >  1 file changed, 1 insertion(+), 4 deletions(-)
> > 
> > diff --git a/fs/btrfs/root-tree.c b/fs/btrfs/root-tree.c
> > index 702dc5441f03..ec9e78f65fca 100644
> > --- a/fs/btrfs/root-tree.c
> > +++ b/fs/btrfs/root-tree.c
> > @@ -39,10 +39,7 @@ static void btrfs_read_root_item(struct extent_buffer *eb, int slot,
> >  		need_reset = 1;
> >  	}
> >  	if (need_reset) {
> > -		memset(&item->generation_v2, 0,
> > -			sizeof(*item) - offsetof(struct btrfs_root_item,
> > -					generation_v2));
> > -
> 
> Please add
> 		/* Clear all members from generation_v2 onwards */
> 
> > +		memset_after(item, 0, level);

Perhaps there should be another helper memset_starting()? That would
make these cases a bit more self-documenting.

+		memset_starting(item, 0, generation_v2);

> >  		generate_random_guid(item->uuid);
> 
> Acked-by: David Sterba <[email-masked]>

What do you think?

-Kees

-- 
Kees Cook