wifi: wext: use flex array destination for memcpy()

Syzkaller reports buffer overflow false positive as follows:
------------[ cut here ]------------
memcpy: detected field-spanning write (size 8) of single field
	"&compat_event->pointer" at net/wireless/wext-core.c:623 (size 4)
WARNING: CPU: 0 PID: 3607 at net/wireless/wext-core.c:623
	wireless_send_event+0xab5/0xca0 net/wireless/wext-core.c:623
Modules linked in:
CPU: 1 PID: 3607 Comm: syz-executor659 Not tainted
	6.0.0-rc6-next-20220921-syzkaller #0
[...]
Call Trace:
 <TASK>
 ioctl_standard_call+0x155/0x1f0 net/wireless/wext-core.c:1022
 wireless_process_ioctl+0xc8/0x4c0 net/wireless/wext-core.c:955
 wext_ioctl_dispatch net/wireless/wext-core.c:988 [inline]
 wext_ioctl_dispatch net/wireless/wext-core.c:976 [inline]
 wext_handle_ioctl+0x26b/0x280 net/wireless/wext-core.c:1049
 sock_ioctl+0x285/0x640 net/socket.c:1220
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:870 [inline]
 __se_sys_ioctl fs/ioctl.c:856 [inline]
 __x64_sys_ioctl+0x193/0x200 fs/ioctl.c:856
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd
 [...]
 </TASK>

Wireless events will be sent on the appropriate channels in
wireless_send_event(). Different wireless events may have different
payload structure and size, so kernel uses **len** and **cmd** field
in struct __compat_iw_event as wireless event common LCP part, uses
**pointer** as a label to mark the position of remaining different part.

Yet the problem is that, **pointer** is a compat_caddr_t type, which may
be smaller than the relative structure at the same position. So during
wireless_send_event() tries to parse the wireless events payload, it may
trigger the memcpy() run-time destination buffer bounds checking when the
relative structure's data is copied to the position marked by **pointer**.

This patch solves it by introducing flexible-array field **ptr_bytes**,
to mark the position of the wireless events remaining part next to
LCP part. What's more, this patch also adds **ptr_len** variable in
wireless_send_event() to improve its maintainability.

Reported-and-tested-by: syzbot+473754e5af963cf014cf@syzkaller.appspotmail.com
Link: https://lore.kernel.org/all/00000000000070db2005e95a5984@google.com/Suggested-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Hawkins Jiawei <yin31149@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

include/linux/wireless.h

@@ -26,7 +26,15 @@ struct compat_iw_point {
 struct __compat_iw_event {
 	__u16		len;			/* Real length of this stuff */
 	__u16		cmd;			/* Wireless IOCTL */
-	compat_caddr_t	pointer;
+
+	union {
+		compat_caddr_t	pointer;
+
+		/* we need ptr_bytes to make memcpy() run-time destination
+		 * buffer bounds checking happy, nothing special
+		 */
+		DECLARE_FLEX_ARRAY(__u8, ptr_bytes);
+	};
 };
 #define IW_EV_COMPAT_LCP_LEN offsetof(struct __compat_iw_event, pointer)
 #define IW_EV_COMPAT_POINT_OFF offsetof(struct compat_iw_point, length)

net/wireless/wext-core.c

@@ -468,6 +468,7 @@ void wireless_send_event(struct net_device *	dev,
 	struct __compat_iw_event *compat_event;
 	struct compat_iw_point compat_wrqu;
 	struct sk_buff *compskb;
+	int ptr_len;
 #endif
 
 	/*
@@ -582,6 +583,9 @@ void wireless_send_event(struct net_device *	dev,
 	nlmsg_end(skb, nlh);
 #ifdef CONFIG_COMPAT
 	hdr_len = compat_event_type_size[descr->header_type];
+
+	/* ptr_len is remaining size in event header apart from LCP */
+	ptr_len = hdr_len - IW_EV_COMPAT_LCP_LEN;
 	event_len = hdr_len + extra_len;
 
 	compskb = nlmsg_new(NLMSG_DEFAULT_SIZE, GFP_ATOMIC);
@@ -612,16 +616,15 @@ void wireless_send_event(struct net_device *	dev,
 	if (descr->header_type == IW_HEADER_TYPE_POINT) {
 		compat_wrqu.length = wrqu->data.length;
 		compat_wrqu.flags = wrqu->data.flags;
-		memcpy(&compat_event->pointer,
-			((char *) &compat_wrqu) + IW_EV_COMPAT_POINT_OFF,
-			hdr_len - IW_EV_COMPAT_LCP_LEN);
+		memcpy(compat_event->ptr_bytes,
+		       ((char *)&compat_wrqu) + IW_EV_COMPAT_POINT_OFF,
+			ptr_len);
 		if (extra_len)
-			memcpy(((char *) compat_event) + hdr_len,
-				extra, extra_len);
+			memcpy(&compat_event->ptr_bytes[ptr_len],
+			       extra, extra_len);
 	} else {
 		/* extra_len must be zero, so no if (extra) needed */
-		memcpy(&compat_event->pointer, wrqu,
-			hdr_len - IW_EV_COMPAT_LCP_LEN);
+		memcpy(compat_event->ptr_bytes, wrqu, ptr_len);
 	}
 
 	nlmsg_end(compskb, nlh);