Subject: [PATCH][next] net: dev: Convert sa_data to flexible array in struct sockaddr

From: Kees Cook <[email-masked]>
Date: Tue, 18 Oct 2022 02:56:03 -0700

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: Jakub Kicinski <[email-masked]>
Cc: "David S. Miller" <[email-masked]>
Cc: Eric Dumazet <[email-masked]>
Cc: Paolo Abeni <[email-masked]>
Cc: Jens Axboe <[email-masked]>
Cc: Pavel Begunkov <[email-masked]>
Cc: David Ahern <[email-masked]>
Cc: Dylan Yudaken <[email-masked]>
Cc: Yajun Deng <[email-masked]>
Cc: Petr Machata <[email-masked]>
Cc: Hangbin Liu <[email-masked]>
Cc: Leon Romanovsky <[email-masked]>
Cc: syzbot <[email-masked]>
Cc: Willem de Bruijn <[email-masked]>
Cc: Pablo Neira Ayuso <[email-masked]>
Cc: [email-masked]
Signed-off-by: Kees Cook <[email-masked]>
---
 include/linux/socket.h |  5 ++++-
 net/core/dev.c         |  2 +-
 net/core/dev_ioctl.c   |  2 +-
 net/packet/af_packet.c | 10 +++++-----
 4 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/include/linux/socket.h b/include/linux/socket.h
index de3701a2a212..13c3a237b9c9 100644
--- a/include/linux/socket.h
+++ b/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 {
diff --git a/net/core/dev.c b/net/core/dev.c
index fa53830d0683..0649826e5803 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -8818,7 +8818,7 @@ 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;
 
diff --git a/net/core/dev_ioctl.c b/net/core/dev_ioctl.c
index 7674bb9f3076..5cdbfbf9a7dc 100644
--- a/net/core/dev_ioctl.c
+++ b/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;
diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 6ce8dd19f33c..8c5b3da0c29f 100644
--- a/net/packet/af_packet.c
+++ b/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);
-- 
2.34.1