Re: [syzbot] [net?] [can?] KMSAN: uninit-value in em_canid_match

[shaurya <ssranevjti@gmail.com>]

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#syz test:
git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

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From 137086f25c8aa52e4222355345e4218e410188c5 Mon Sep 17 00:00:00 2001
From: Shaurya Rane <ssrane_b23@ee.vjti.ac.in>
Date: Wed, 26 Nov 2025 02:06:51 +0530
Subject: [PATCH] net/sched: em_canid: add length check before reading CAN ID

Add a check to verify that the skb has at least sizeof(canid_t) bytes
before reading the CAN ID from skb->data. This prevents reading
uninitialized memory when processing malformed packets that don't
contain a valid CAN frame.

Reported-by: syzbot+5d8269a1e099279152bc@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=5d8269a1e099279152bc
Fixes: f057bbb6f9ed ("net: em_canid: Ematch rule to match CAN frames according to their identifiers")
Signed-off-by: Shaurya Rane <ssrane_b23@ee.vjti.ac.in>
---
 net/sched/em_canid.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/sched/em_canid.c b/net/sched/em_canid.c
index 5337bc462755..a9b6cab70ff1 100644
--- a/net/sched/em_canid.c
+++ b/net/sched/em_canid.c
@@ -99,6 +99,9 @@ static int em_canid_match(struct sk_buff *skb, struct tcf_ematch *m,
 	int i;
 	const struct can_filter *lp;
 
+	if (skb->len < sizeof(canid_t))
+		return 0;
+
 	can_id = em_canid_get_id(skb);
 
 	if (can_id & CAN_EFF_FLAG) {
-- 
2.34.1