net/sched: cls_u32: use skb_header_pointer_careful()

jira VULN-176126
cve CVE-2026-23204
commit-author Eric Dumazet <edumazet@google.com>
commit cabd1a9

skb_header_pointer() does not fully validate negative @offset values.

Use skb_header_pointer_careful() instead.

GangMin Kim provided a report and a repro fooling u32_classify():

BUG: KASAN: slab-out-of-bounds in u32_classify+0x1180/0x11b0
net/sched/cls_u32.c:221

Fixes: fbc2e7d ("cls_u32: use skb_header_pointer() to dereference data safely")
	Reported-by: GangMin Kim <km.kim1503@gmail.com>
Closes: https://lore.kernel.org/netdev/CANn89iJkyUZ=mAzLzC4GdcAgLuPnUoivdLaOs6B9rq5_erj76w@mail.gmail.com/T/
	Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260128141539.3404400-3-edumazet@google.com
	Signed-off-by: Jakub Kicinski <kuba@kernel.org>
(cherry picked from commit cabd1a9)
	Signed-off-by: Roxana Nicolescu <rnicolescu@ciq.com>

Author: roxanan1996

net/sched/cls_u32.c

@@ -157,10 +157,8 @@ static int u32_classify(struct sk_buff *skb, const struct tcf_proto *tp,
 			int toff = off + key->off + (off2 & key->offmask);
 			__be32 *data, hdata;
 
-			if (skb_headroom(skb) + toff > INT_MAX)
-				goto out;
-
-			data = skb_header_pointer(skb, toff, 4, &hdata);
+			data = skb_header_pointer_careful(skb, toff, 4,
+							  &hdata);
 			if (!data)
 				goto out;
 			if ((*data ^ key->val) & key->mask) {
@@ -212,8 +210,9 @@ static int u32_classify(struct sk_buff *skb, const struct tcf_proto *tp,
 		if (ht->divisor) {
 			__be32 *data, hdata;
 
-			data = skb_header_pointer(skb, off + n->sel.hoff, 4,
-						  &hdata);
+			data = skb_header_pointer_careful(skb,
+							  off + n->sel.hoff,
+							  4, &hdata);
 			if (!data)
 				goto out;
 			sel = ht->divisor & u32_hash_fold(*data, &n->sel,
@@ -227,7 +226,7 @@ static int u32_classify(struct sk_buff *skb, const struct tcf_proto *tp,
 			if (n->sel.flags & TC_U32_VAROFFSET) {
 				__be16 *data, hdata;
 
-				data = skb_header_pointer(skb,
+				data = skb_header_pointer_careful(skb,
 							  off + n->sel.offoff,
 							  2, &hdata);
 				if (!data)