linux-kernel - Re: [PATCH 47/64] btrfs: Use memset

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Message-ID: <202107310822.31BEB6E543@keescook>
Date:   Sat, 31 Jul 2021 08:25:51 -0700
From:   Kees Cook <keescook@...omium.org>
To:     dsterba@...e.cz, linux-hardening@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Keith Packard <keithpac@...zon.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-wireless@...r.kernel.org,
        netdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-staging@...ts.linux.dev, linux-block@...r.kernel.org,
        linux-kbuild@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH 47/64] btrfs: Use memset_after() to clear end of struct

On Thu, Jul 29, 2021 at 12:33:37PM +0200, David Sterba wrote:
> On Wed, Jul 28, 2021 at 02:56:31PM -0700, Kees Cook wrote:
> > 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 <keescook@...omium.org>
> > > > ---
> > > >  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.
> 
> That would be better, yes.
> 
> > +		memset_starting(item, 0, generation_v2);
> 
> memset_from?

For v2, I bikeshed this to "memset_startat" since "from" is semantically
close to "source" which I thought might be confusing. (I, too, did not
like "starting".) :)

Can I make "bikeshed" a verb? :P

-- 
Kees Cook