[PATCH v3] net/sched: em_canid: fix uninit-value in em_canid_match

[ssrane_b23]

From: Shaurya Rane <ssrane_b23@xxxxxxxxxxxxx>

Use pskb_may_pull() to ensure a complete CAN frame is present in the
linear data buffer before reading the CAN ID. A simple skb->len check
is insufficient because it only verifies the total data length but does
not guarantee the data is present in skb->data (it could be in
fragments).

pskb_may_pull() both validates the length and pulls fragmented data
into the linear buffer if necessary, making it safe to directly
access skb->data.

Reported-by: syzbot+5d8269a1e099279152bc@xxxxxxxxxxxxxxxxxxxxxxxxx
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@xxxxxxxxxxxxx>
---
v3: Use CAN_MTU to validate a complete CAN frame is present
v2: Use pskb_may_pull() instead of skb->len check to properly
    handle fragmented skbs
---
 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..2d27f91d8441 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 (!pskb_may_pull(skb, CAN_MTU))
+		return 0;
+
 	can_id = em_canid_get_id(skb);
 
 	if (can_id & CAN_EFF_FLAG) {
-- 
2.34.1