[PATCH v2] ath9k: ath9k_htc_rx_msg: return when sk_buff is too small

At the start of ath9k_htc_rx_msg, we check to see if the skb pointer is
valid, but what we don't do is check if it is large enough to contain a
valid struct htc_frame_hdr. We should check for this and return if not,
as the buffer is invalid in this case. Fixes a KMSAN-found uninit-value bug
reported by syzbot at:
https://syzkaller.appspot.com/bug?id=7dccb7d9ad4251df1c49f370607a49e1f09644ee

Reported-by: syzbot+e4534e8c1c382508312c@syzkaller.appspotmail.com
Signed-off-by: Phillip Potter <phil@philpotter.co.uk>
---

V2:
* Free skb properly when this problem is detected, as pointed out by
  Florian Westphal.

---
 drivers/net/wireless/ath/ath9k/htc_hst.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/net/wireless/ath/ath9k/htc_hst.c b/drivers/net/wireless/ath/ath9k/htc_hst.c
index 510e61e97dbc..1fe89b068ac4 100644
--- a/drivers/net/wireless/ath/ath9k/htc_hst.c
+++ b/drivers/net/wireless/ath/ath9k/htc_hst.c

@@ -406,6 +406,11 @@ void ath9k_htc_rx_msg(struct htc_target *htc_handle,
 	if (!htc_handle || !skb)
 		return;
 
+	if (!pskb_may_pull(skb, sizeof(struct htc_frame_hdr))) {
+		kfree_skb(skb);
+		return;
+	}
+
 	htc_hdr = (struct htc_frame_hdr *) skb->data;
 	epid = htc_hdr->endpoint_id;
 

-- 
2.30.2