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

Kees Cook
2021-07-30T01:42:02Z

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@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?
>
> 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