wifi: cfg80211: check A-MSDU format more carefully

If it looks like there's another subframe in the A-MSDU
but the header isn't fully there, we can end up reading
data out of bounds, only to discard later. Make this a
bit more careful and check if the subframe header can
even be present.

Reported-by: syzbot+d050d437fe47d479d210@syzkaller.appspotmail.com
Link: https://msgid.link/20240226203405.a731e2c95e38.I82ce7d8c0cc8970ce29d0a39fdc07f1ffc425be4@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/util.c

@@ -791,15 +791,19 @@ ieee80211_amsdu_subframe_length(void *field, u8 mesh_flags, u8 hdr_type)
 
 bool ieee80211_is_valid_amsdu(struct sk_buff *skb, u8 mesh_hdr)
 {
-	int offset = 0, remaining, subframe_len, padding;
+	int offset = 0, subframe_len, padding;
 
 	for (offset = 0; offset < skb->len; offset += subframe_len + padding) {
+		int remaining = skb->len - offset;
 		struct {
 		    __be16 len;
 		    u8 mesh_flags;
 		} hdr;
 		u16 len;
 
+		if (sizeof(hdr) > remaining)
+			return false;
+
 		if (skb_copy_bits(skb, offset + 2 * ETH_ALEN, &hdr, sizeof(hdr)) < 0)
 			return false;
 
@@ -807,7 +811,6 @@ bool ieee80211_is_valid_amsdu(struct sk_buff *skb, u8 mesh_hdr)
 						      mesh_hdr);
 		subframe_len = sizeof(struct ethhdr) + len;
 		padding = (4 - subframe_len) & 0x3;
-		remaining = skb->len - offset;
 
 		if (subframe_len > remaining)
 			return false;
@@ -825,7 +828,7 @@ void ieee80211_amsdu_to_8023s(struct sk_buff *skb, struct sk_buff_head *list,
 {
 	unsigned int hlen = ALIGN(extra_headroom, 4);
 	struct sk_buff *frame = NULL;
-	int offset = 0, remaining;
+	int offset = 0;
 	struct {
 		struct ethhdr eth;
 		uint8_t flags;
@@ -839,10 +842,14 @@ void ieee80211_amsdu_to_8023s(struct sk_buff *skb, struct sk_buff_head *list,
 		copy_len = sizeof(hdr);
 
 	while (!last) {
+		int remaining = skb->len - offset;
 		unsigned int subframe_len;
 		int len, mesh_len = 0;
 		u8 padding;
 
+		if (copy_len > remaining)
+			goto purge;
+
 		skb_copy_bits(skb, offset, &hdr, copy_len);
 		if (iftype == NL80211_IFTYPE_MESH_POINT)
 			mesh_len = __ieee80211_get_mesh_hdrlen(hdr.flags);
@@ -852,7 +859,6 @@ void ieee80211_amsdu_to_8023s(struct sk_buff *skb, struct sk_buff_head *list,
 		padding = (4 - subframe_len) & 0x3;
 
 		/* the last MSDU has no padding */
-		remaining = skb->len - offset;
 		if (subframe_len > remaining)
 			goto purge;
 		/* mitigate A-MSDU aggregation injection attacks */