[PATCH] ppp: Prevent out-of-bounds access in ppp_sync_txmunge

Fix an issue detected by syzbot with KMSAN:

BUG: KMSAN: uninit-value in ppp_sync_txmunge
drivers/net/ppp/ppp_synctty.c:516 [inline]
BUG: KMSAN: uninit-value in ppp_sync_send+0x21c/0xb00
drivers/net/ppp/ppp_synctty.c:568

Ensure sk_buff is valid and has at least 3 bytes before accessing its
data field in ppp_sync_txmunge(). Without this check, the function may
attempt to read uninitialized or invalid memory, leading to undefined
behavior.

To address this, add a validation check at the beginning of the function
to safely handle cases where skb is NULL or too small. If either condition
is met, free the skb and return NULL to prevent processing an invalid
packet.

Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=29fc8991b0ecb186cf40
Tested-by: [email protected]
Signed-off-by: Purva Yeshi <[email protected]>
---
 drivers/net/ppp/ppp_synctty.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/net/ppp/ppp_synctty.c b/drivers/net/ppp/ppp_synctty.c
index 644e99fc3..e537ea3d9 100644
--- a/drivers/net/ppp/ppp_synctty.c
+++ b/drivers/net/ppp/ppp_synctty.c
@@ -506,6 +506,12 @@ ppp_sync_txmunge(struct syncppp *ap, struct sk_buff *skb)
 	unsigned char *data;
 	int islcp;
 
+	/* Ensure skb is not NULL and has at least 3 bytes */
+	if (!skb || skb->len < 3) {
+		kfree_skb(skb);
+		return NULL;
+	}
+
 	data  = skb->data;
 	proto = get_unaligned_be16(data);
 
-- 
2.34.1