linux-kernel - Re: [PATCH 62/64] netlink: Avoid false-positive memcpy() warning

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Date:   Thu, 29 Jul 2021 18:41:54 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-hardening@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Keith Packard <keithpac@...zon.com>,
        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 62/64] netlink: Avoid false-positive memcpy() warning

On Wed, Jul 28, 2021 at 07:49:46AM +0200, 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@...omium.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?
> 
> Nothing breaks?

It really shouldn't break anything. Adding a flex array doesn't change
the size. And with Rasmus's suggestion (naming it "nlmsg_content") it
should be safe against weird global macro collisions, etc.

-- 
Kees Cook