netdev - [PATCH net-next v1 1/1] ppp: Fix KMSAN warning by initializing 2-byte header

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Message-ID: <20250218133145.265313-2-jiayuan.chen@linux.dev>
Date: Tue, 18 Feb 2025 21:31:44 +0800
From: Jiayuan Chen <jiayuan.chen@...ux.dev>
To: bpf@...r.kernel.org,
	netdev@...r.kernel.org
Cc: andrew+netdev@...n.ch,
	davem@...emloft.ne,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com,
	horms@...nel.org,
	ricardo@...liere.net,
	jiayuan.chen@...ux.dev,
	viro@...iv.linux.org.uk,
	dmantipov@...dex.ru,
	aleksander.lobakin@...el.com,
	linux-ppp@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	mrpre@....com,
	syzbot+853242d9c9917165d791@...kaller.appspotmail.com
Subject: [PATCH net-next v1 1/1] ppp: Fix KMSAN warning by initializing 2-byte header

The ppp program adds a 2-byte pseudo-header for socket filters, which is
normally skipped by regular BPF programs, causing no issues.

However, for abnormal BPF programs that use these uninitialized 2 bytes,
a KMSAN warning is triggered.

Reported-by: syzbot+853242d9c9917165d791@...kaller.appspotmail.com
Closes: https://lore.kernel.org/bpf/000000000000dea025060d6bc3bc@google.com/
Signed-off-by: Jiayuan Chen <jiayuan.chen@...ux.dev>
---
 drivers/net/ppp/ppp_generic.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index 4583e15ad03a..a913403d5847 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -1765,7 +1765,7 @@ ppp_send_frame(struct ppp *ppp, struct sk_buff *skb)
 		/* check if we should pass this packet */
 		/* the filter instructions are constructed assuming
 		   a four-byte PPP header on each packet */
-		*(u8 *)skb_push(skb, 2) = 1;
+		*(u16 *)skb_push(skb, 2) = 1;
 		if (ppp->pass_filter &&
 		    bpf_prog_run(ppp->pass_filter, skb) == 0) {
 			if (ppp->debug & 1)
@@ -2489,7 +2489,7 @@ ppp_receive_nonmp_frame(struct ppp *ppp, struct sk_buff *skb)
 			if (skb_unclone(skb, GFP_ATOMIC))
 				goto err;
 
-			*(u8 *)skb_push(skb, 2) = 0;
+			*(u16 *)skb_push(skb, 2) = 0;
 			if (ppp->pass_filter &&
 			    bpf_prog_run(ppp->pass_filter, skb) == 0) {
 				if (ppp->debug & 1)
-- 
2.47.1