Subject: Re: [PATCH] Add linux-next specific files for 20220923

From: Kees Cook <[email-masked]>
Date: Sat, 24 Sep 2022 09:26:01 -0700

On Sat, Sep 24, 2022 at 11:55:14PM +0800, Hawkins Jiawei wrote:
> > And as for the value of offsetof in calculating **offset**,
> > I wonder if we can use the macro defined in
> > include/linux/wireless.h as below, which makes code simplier:
> > #define IW_EV_COMPAT_LCP_LEN offsetof(struct __compat_iw_event, pointer)

Ah yes, that would be good.

> According to above code, it seems that kernel will saves enough memory
> (hdr_len + extra_len bytes) for payload structure in
> nla_reserve()(Please correct me if I am wrong), pointed by compat_event.
> So I wonder if we can use unsafe_memcpy(), to avoid unnecessary
> memcpy() check as below, which seems more simple:

I'd rather this was properly resolved with the creation of a real
flexible array so that when bounds tracking gets improved in the future,
the compiler can reason about it better. And, I think, it makes the code
more readable:

diff --git a/include/linux/wireless.h b/include/linux/wireless.h
index 2d1b54556eff..e0b4b46da63f 100644
--- a/include/linux/wireless.h
+++ b/include/linux/wireless.h
@@ -26,7 +26,10 @@ 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;
+		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)
diff --git a/net/wireless/wext-core.c b/net/wireless/wext-core.c
index 76a80a41615b..6079c8f4b634 100644
--- a/net/wireless/wext-core.c
+++ b/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 +583,7 @@ 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 = hdr_len - IW_EV_COMPAT_LCP_LEN;
 	event_len = hdr_len + extra_len;
 
 	compskb = nlmsg_new(NLMSG_DEFAULT_SIZE, GFP_ATOMIC);
@@ -612,16 +614,15 @@ 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,
+		memcpy(compat_event->ptr_bytes,
 			((char *) &compat_wrqu) + IW_EV_COMPAT_POINT_OFF,
-			hdr_len - IW_EV_COMPAT_LCP_LEN);
+			ptr_len);
 		if (extra_len)
-			memcpy(((char *) compat_event) + hdr_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);

-- 
Kees Cook