[PATCH net 2/9] netfilter: xt_u32: reject invalid shift counts

[Florian Westphal <fw@strlen.de>]

From: Wyatt Feng <bronzed_45_vested@icloud.com>

u32_match_it() executes rule-supplied shift operands on a 32-bit
value. A malformed u32 rule can provide a shift count of 32 or more,
triggering an undefined shift out-of-bounds during packet evaluation.

Validate XT_U32_LEFTSH and XT_U32_RIGHTSH operands in
u32_mt_checkentry() and reject malformed rules before they reach the
packet path.

Fixes: 1b50b8a371e9 ("[NETFILTER]: Add u32 match")
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Reported-by: Zhengchuan Liang <zcliangcn@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Assisted-by: Codex:GPT-5.4
Signed-off-by: Wyatt Feng <bronzed_45_vested@icloud.com>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
Signed-off-by: Florian Westphal <fw@strlen.de>
---
 net/netfilter/xt_u32.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/net/netfilter/xt_u32.c b/net/netfilter/xt_u32.c
index 117d4615d668..ec1a21e3b6e2 100644
--- a/net/netfilter/xt_u32.c
+++ b/net/netfilter/xt_u32.c
@@ -100,7 +100,7 @@ static int u32_mt_checkentry(const struct xt_mtchk_param *par)
 {
 	const struct xt_u32 *data = par->matchinfo;
 	const struct xt_u32_test *ct;
-	unsigned int i;
+	unsigned int i, j;
 
 	if (data->ntests > ARRAY_SIZE(data->tests))
 		return -EINVAL;
@@ -111,6 +111,16 @@ static int u32_mt_checkentry(const struct xt_mtchk_param *par)
 		if (ct->nnums > ARRAY_SIZE(ct->location) ||
 		    ct->nvalues > ARRAY_SIZE(ct->value))
 			return -EINVAL;
+
+		for (j = 1; j < ct->nnums; ++j) {
+			switch (ct->location[j].nextop) {
+			case XT_U32_LEFTSH:
+			case XT_U32_RIGHTSH:
+				if (ct->location[j].number >= 32)
+					return -EINVAL;
+				break;
+			}
+		}
 	}
 
 	return 0;
-- 
2.54.0