netlink: Use copy_to_user() for optval in netlink_getsockopt().

Brad Spencer provided a detailed report [0] that when calling getsockopt()
for AF_NETLINK, some SOL_NETLINK options set only 1 byte even though such
options require at least sizeof(int) as length.

The options return a flag value that fits into 1 byte, but such behaviour
confuses users who do not initialise the variable before calling
getsockopt() and do not strictly check the returned value as char.

Currently, netlink_getsockopt() uses put_user() to copy data to optlen and
optval, but put_user() casts the data based on the pointer, char *optval.
As a result, only 1 byte is set to optval.

To avoid this behaviour, we need to use copy_to_user() or cast optval for
put_user().

Note that this changes the behaviour on big-endian systems, but we document
that the size of optval is int in the man page.

  $ man 7 netlink
  ...
  Socket options
       To set or get a netlink socket option, call getsockopt(2) to read
       or setsockopt(2) to write the option with the option level argument
       set to SOL_NETLINK.  Unless otherwise noted, optval is a pointer to
       an int.

Fixes: 9a4595bc ("[NETLINK]: Add set/getsockopt options to support more than 32 groups")
Fixes: be0c22a4 ("netlink: add NETLINK_BROADCAST_ERROR socket option")
Fixes: 38938bfe ("netlink: add NETLINK_NO_ENOBUFS socket flag")
Fixes: 0a6a3a23 ("netlink: add NETLINK_CAP_ACK socket option")
Fixes: 2d4bc933 ("netlink: extended ACK reporting")
Fixes: 89d35528 ("netlink: Add new socket option to enable strict checking on dumps")
Reported-by: Brad Spencer <bspencer@blackberry.com>
Link: https://lore.kernel.org/netdev/ZD7VkNWFfp22kTDt@datsun.rim.net/Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Link: https://lore.kernel.org/r/20230421185255.94606-1-kuniyu@amazon.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/netlink/af_netlink.c

@@ -1742,7 +1742,8 @@ static int netlink_getsockopt(struct socket *sock, int level, int optname,
 {
 	struct sock *sk = sock->sk;
 	struct netlink_sock *nlk = nlk_sk(sk);
-	int len, val, err;
+	unsigned int flag;
+	int len, val;
 
 	if (level != SOL_NETLINK)
 		return -ENOPROTOOPT;
@@ -1754,39 +1755,17 @@ static int netlink_getsockopt(struct socket *sock, int level, int optname,
 
 	switch (optname) {
 	case NETLINK_PKTINFO:
-		if (len < sizeof(int))
-			return -EINVAL;
-		len = sizeof(int);
-		val = nlk->flags & NETLINK_F_RECV_PKTINFO ? 1 : 0;
-		if (put_user(len, optlen) ||
-		    put_user(val, optval))
-			return -EFAULT;
-		err = 0;
+		flag = NETLINK_F_RECV_PKTINFO;
 		break;
 	case NETLINK_BROADCAST_ERROR:
-		if (len < sizeof(int))
-			return -EINVAL;
-		len = sizeof(int);
-		val = nlk->flags & NETLINK_F_BROADCAST_SEND_ERROR ? 1 : 0;
-		if (put_user(len, optlen) ||
-		    put_user(val, optval))
-			return -EFAULT;
-		err = 0;
+		flag = NETLINK_F_BROADCAST_SEND_ERROR;
 		break;
 	case NETLINK_NO_ENOBUFS:
-		if (len < sizeof(int))
-			return -EINVAL;
-		len = sizeof(int);
-		val = nlk->flags & NETLINK_F_RECV_NO_ENOBUFS ? 1 : 0;
-		if (put_user(len, optlen) ||
-		    put_user(val, optval))
-			return -EFAULT;
-		err = 0;
+		flag = NETLINK_F_RECV_NO_ENOBUFS;
 		break;
 	case NETLINK_LIST_MEMBERSHIPS: {
-		int pos, idx, shift;
+		int pos, idx, shift, err = 0;
 
-		err = 0;
 		netlink_lock_table();
 		for (pos = 0; pos * 8 < nlk->ngroups; pos += sizeof(u32)) {
 			if (len - pos < sizeof(u32))
@@ -1803,40 +1782,32 @@ static int netlink_getsockopt(struct socket *sock, int level, int optname,
 		if (put_user(ALIGN(nlk->ngroups / 8, sizeof(u32)), optlen))
 			err = -EFAULT;
 		netlink_unlock_table();
-		break;
+		return err;
 	}
 	case NETLINK_CAP_ACK:
-		if (len < sizeof(int))
-			return -EINVAL;
-		len = sizeof(int);
-		val = nlk->flags & NETLINK_F_CAP_ACK ? 1 : 0;
-		if (put_user(len, optlen) ||
-		    put_user(val, optval))
-			return -EFAULT;
-		err = 0;
+		flag = NETLINK_F_CAP_ACK;
 		break;
 	case NETLINK_EXT_ACK:
-		if (len < sizeof(int))
-			return -EINVAL;
-		len = sizeof(int);
-		val = nlk->flags & NETLINK_F_EXT_ACK ? 1 : 0;
-		if (put_user(len, optlen) || put_user(val, optval))
-			return -EFAULT;
-		err = 0;
+		flag = NETLINK_F_EXT_ACK;
 		break;
 	case NETLINK_GET_STRICT_CHK:
+		flag = NETLINK_F_STRICT_CHK;
+		break;
+	default:
+		return -ENOPROTOOPT;
+	}
+
 	if (len < sizeof(int))
 		return -EINVAL;
+
 	len = sizeof(int);
-		val = nlk->flags & NETLINK_F_STRICT_CHK ? 1 : 0;
-		if (put_user(len, optlen) || put_user(val, optval))
+	val = nlk->flags & flag ? 1 : 0;
+
+	if (put_user(len, optlen) ||
+	    copy_to_user(optval, &val, len))
 		return -EFAULT;
-		err = 0;
-		break;
-	default:
-		err = -ENOPROTOOPT;
-	}
-	return err;
+
+	return 0;
 }
 
 static void netlink_cmsg_recv_pktinfo(struct msghdr *msg, struct sk_buff *skb)