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Date:   Thu, 29 Jul 2021 18:39:22 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-hardening@vger.kernel.org,
        "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        Keith Packard <keithpac@amazon.com>,
        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 62/64] netlink: Avoid false-positive memcpy() warning
Message-ID: <202107291838.25D1F118C@keescook>
References: <20210727205855.411487-1-keescook@chromium.org>
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 <YQDv+oG7ok0T1L+r@kroah.com>
 <d7251d92-150b-5346-6237-52afc154bb00@rasmusvillemoes.dk>
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On Wed, Jul 28, 2021 at 01:24:01PM +0200, Rasmus Villemoes wrote:
> On 28/07/2021 07.49, Greg Kroah-Hartman wrote:
> > On Tue, Jul 27, 2021 at 01:58:53PM -0700, Kees Cook wrote:
> >> In preparation for FORTIFY_SOURCE performing compile-time and run-time
> >> field bounds checking for memcpy(), memmove(), and memset(), avoid
> >> intentionally writing across neighboring fields.
> >>
> >> Add a flexible array member to mark the end of struct nlmsghdr, and
> >> split the memcpy() to avoid false positive memcpy() warning:
> >>
> >> memcpy: detected field-spanning write (size 32) of single field (size 16)
> >>
> >> Signed-off-by: Kees Cook <keescook@chromium.org>
> >> ---
> >>  include/uapi/linux/netlink.h | 1 +
> >>  net/netlink/af_netlink.c     | 4 +++-
> >>  2 files changed, 4 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/include/uapi/linux/netlink.h b/include/uapi/linux/netlink.h
> >> index 4c0cde075c27..ddeaa748df5e 100644
> >> --- a/include/uapi/linux/netlink.h
> >> +++ b/include/uapi/linux/netlink.h
> >> @@ -47,6 +47,7 @@ struct nlmsghdr {
> >>  	__u16		nlmsg_flags;	/* Additional flags */
> >>  	__u32		nlmsg_seq;	/* Sequence number */
> >>  	__u32		nlmsg_pid;	/* Sending process port ID */
> >> +	__u8		contents[];
> > 
> > Is this ok to change a public, userspace visable, structure?
> 
> At least it should keep using a nlmsg_ prefix for consistency and reduce
> risk of collision with somebody having defined an object-like contents
> macro. But there's no guarantees in any case, of course.

Ah, good call. I've adjusted this and added a comment.

Thanks!

-Kees

-- 
Kees Cook