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

Kees Cook
2021-07-30T01:39:32Z

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