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Date:   Wed, 28 Jul 2021 11:42:15 +0200
From:   David Sterba <dsterba@suse.cz>
To:     Kees Cook <keescook@chromium.org>
Cc:     linux-hardening@vger.kernel.org,
        "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        Keith Packard <keithpac@amazon.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-wireless@vger.kernel.org,
        netdev@vger.kernel.org, dri-devel@lists.freedesktop.org,
        linux-staging@lists.linux.dev, linux-block@vger.kernel.org,
        linux-kbuild@vger.kernel.org, clang-built-linux@googlegroups.com
Subject: Re: [PATCH 47/64] btrfs: Use memset_after() to clear end of struct
Message-ID: <20210728094215.GX5047@twin.jikos.cz>
Reply-To: dsterba@suse.cz
Mail-Followup-To: dsterba@suse.cz, Kees Cook <keescook@chromium.org>,
        linux-hardening@vger.kernel.org,
        "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        Keith Packard <keithpac@amazon.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-wireless@vger.kernel.org,
        netdev@vger.kernel.org, dri-devel@lists.freedesktop.org,
        linux-staging@lists.linux.dev, linux-block@vger.kernel.org,
        linux-kbuild@vger.kernel.org, clang-built-linux@googlegroups.com
References: <20210727205855.411487-1-keescook@chromium.org>
 <20210727205855.411487-48-keescook@chromium.org>
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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@chromium.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);
>  		generate_random_guid(item->uuid);

Acked-by: David Sterba <dsterba@suse.com>