net: fix rtnh_ok()

BugLink: http://bugs.launchpad.net/bugs/1774173

commit b1993a2d upstream.

syzbot reported :

BUG: KMSAN: uninit-value in rtnh_ok include/net/nexthop.h:11 [inline]
BUG: KMSAN: uninit-value in fib_count_nexthops net/ipv4/fib_semantics.c:469 [inline]
BUG: KMSAN: uninit-value in fib_create_info+0x554/0x8d20 net/ipv4/fib_semantics.c:1091

@remaining is an integer, coming from user space.
If it is negative we want rtnh_ok() to return false.

Fixes: 4e902c57 ("[IPv4]: FIB configuration using struct fib_config")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

include/net/nexthop.h

@@ -6,7 +6,7 @@
 
 static inline int rtnh_ok(const struct rtnexthop *rtnh, int remaining)
 {
-	return remaining >= sizeof(*rtnh) &&
+	return remaining >= (int)sizeof(*rtnh) &&
 	       rtnh->rtnh_len >= sizeof(*rtnh) &&
 	       rtnh->rtnh_len <= remaining;
 }