net: dev: Convert sa_data to flexible array in struct sockaddr

One of the worst offenders of "fake flexible arrays" is struct sockaddr,
as it is the classic example of why GCC and Clang have been traditionally
forced to treat all trailing arrays as fake flexible arrays: in the
distant misty past, sa_data became too small, and code started just
treating it as a flexible array, even though it was fixed-size. The
special case by the compiler is specifically that sizeof(sa->sa_data)
and FORTIFY_SOURCE (which uses __builtin_object_size(sa->sa_data, 1))
do not agree (14 and -1 respectively), which makes FORTIFY_SOURCE treat
it as a flexible array.

However, the coming -fstrict-flex-arrays compiler flag will remove
these special cases so that FORTIFY_SOURCE can gain coverage over all
the trailing arrays in the kernel that are _not_ supposed to be treated
as a flexible array. To deal with this change, convert sa_data to a true
flexible array. To keep the structure size the same, move sa_data into
a union with a newly introduced sa_data_min with the original size. The
result is that FORTIFY_SOURCE can continue to have no idea how large
sa_data may actually be, but anything using sizeof(sa->sa_data) must
switch to sizeof(sa->sa_data_min).

Cc: Jens Axboe <axboe@kernel.dk>
Cc: Pavel Begunkov <asml.silence@gmail.com>
Cc: David Ahern <dsahern@kernel.org>
Cc: Dylan Yudaken <dylany@fb.com>
Cc: Yajun Deng <yajun.deng@linux.dev>
Cc: Petr Machata <petrm@nvidia.com>
Cc: Hangbin Liu <liuhangbin@gmail.com>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: syzbot <syzkaller@googlegroups.com>
Cc: Willem de Bruijn <willemb@google.com>
Cc: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20221018095503.never.671-kees@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/socket.h

@@ -33,7 +33,10 @@ typedef __kernel_sa_family_t	sa_family_t;
 
 struct sockaddr {
 	sa_family_t	sa_family;	/* address family, AF_xxx	*/
-	char		sa_data[14];	/* 14 bytes of protocol address	*/
+	union {
+		char sa_data_min[14];		/* Minimum 14 bytes of protocol address	*/
+		DECLARE_FLEX_ARRAY(char, sa_data);
+	};
 };
 
 struct linger {

net/core/dev.c

@@ -8822,7 +8822,7 @@ EXPORT_SYMBOL(dev_set_mac_address_user);
 
 int dev_get_mac_address(struct sockaddr *sa, struct net *net, char *dev_name)
 {
-	size_t size = sizeof(sa->sa_data);
+	size_t size = sizeof(sa->sa_data_min);
 	struct net_device *dev;
 	int ret = 0;
 

net/core/dev_ioctl.c

@@ -342,7 +342,7 @@ static int dev_ifsioc(struct net *net, struct ifreq *ifr, void __user *data,
 		if (ifr->ifr_hwaddr.sa_family != dev->type)
 			return -EINVAL;
 		memcpy(dev->broadcast, ifr->ifr_hwaddr.sa_data,
-		       min(sizeof(ifr->ifr_hwaddr.sa_data),
+		       min(sizeof(ifr->ifr_hwaddr.sa_data_min),
 			   (size_t)dev->addr_len));
 		call_netdevice_notifiers(NETDEV_CHANGEADDR, dev);
 		return 0;

net/packet/af_packet.c

@@ -3277,7 +3277,7 @@ static int packet_bind_spkt(struct socket *sock, struct sockaddr *uaddr,
 			    int addr_len)
 {
 	struct sock *sk = sock->sk;
-	char name[sizeof(uaddr->sa_data) + 1];
+	char name[sizeof(uaddr->sa_data_min) + 1];
 
 	/*
 	 *	Check legality
@@ -3288,8 +3288,8 @@ static int packet_bind_spkt(struct socket *sock, struct sockaddr *uaddr,
 	/* uaddr->sa_data comes from the userspace, it's not guaranteed to be
 	 * zero-terminated.
 	 */
-	memcpy(name, uaddr->sa_data, sizeof(uaddr->sa_data));
-	name[sizeof(uaddr->sa_data)] = 0;
+	memcpy(name, uaddr->sa_data, sizeof(uaddr->sa_data_min));
+	name[sizeof(uaddr->sa_data_min)] = 0;
 
 	return packet_do_bind(sk, name, 0, pkt_sk(sk)->num);
 }
@@ -3561,11 +3561,11 @@ static int packet_getname_spkt(struct socket *sock, struct sockaddr *uaddr,
 		return -EOPNOTSUPP;
 
 	uaddr->sa_family = AF_PACKET;
-	memset(uaddr->sa_data, 0, sizeof(uaddr->sa_data));
+	memset(uaddr->sa_data, 0, sizeof(uaddr->sa_data_min));
 	rcu_read_lock();
 	dev = dev_get_by_index_rcu(sock_net(sk), READ_ONCE(pkt_sk(sk)->ifindex));
 	if (dev)
-		strscpy(uaddr->sa_data, dev->name, sizeof(uaddr->sa_data));
+		strscpy(uaddr->sa_data, dev->name, sizeof(uaddr->sa_data_min));
 	rcu_read_unlock();
 
 	return sizeof(*uaddr);