netdev - [PATCH net-next v4 1/1] ppp: Fix KMSAN warning by initializing 2-byte header

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Message-ID: <20250226013658.891214-2-jiayuan.chen@linux.dev>
Date: Wed, 26 Feb 2025 09:36:58 +0800
From: Jiayuan Chen <jiayuan.chen@...ux.dev>
To: horms@...nel.org,
	kuba@...nel.org
Cc: bpf@...r.kernel.org,
	netdev@...r.kernel.org,
	andrew+netdev@...n.ch,
	davem@...emloft.net,
	edumazet@...gle.com,
	pabeni@...hat.com,
	ricardo@...liere.net,
	viro@...iv.linux.org.uk,
	dmantipov@...dex.ru,
	aleksander.lobakin@...el.com,
	linux-ppp@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	mrpre@....com,
	Jiayuan Chen <jiayuan.chen@...ux.dev>,
	syzbot+853242d9c9917165d791@...kaller.appspotmail.com
Subject: [PATCH net-next v4 1/1] ppp: Fix KMSAN warning by initializing 2-byte header

The PPP driver adds an extra 2-byte header to enable socket filters to run
correctly. However, the driver only initializes the first byte, which
indicates the direction. For normal BPF programs, this is not a problem
since they only read the first byte.

Nevertheless, for carefully crafted BPF programs, if they read the second
byte, this will trigger a KMSAN warning for reading uninitialized data.

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 | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index 4583e15ad03a..b4433badf03c 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -72,6 +72,10 @@
 #define PPP_PROTO_LEN	2
 #define PPP_LCP_HDRLEN	4
 
+/* These are fields recognized by libpcap */
+#define PPP_FILTER_OUTBOUND_TAG 0x0100
+#define PPP_FILTER_INBOUND_TAG  0x0000
+
 /*
  * An instance of /dev/ppp can be associated with either a ppp
  * interface unit or a ppp channel.  In both cases, file->private_data
@@ -1762,10 +1766,15 @@ ppp_send_frame(struct ppp *ppp, struct sk_buff *skb)
 
 	if (proto < 0x8000) {
 #ifdef CONFIG_PPP_FILTER
-		/* 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;
+		/* Check if we should pass this packet.
+		 * The filter instructions are constructed assuming
+		 * a four-byte PPP header on each packet. The first byte
+		 * indicates the direction, and the second byte is meaningless,
+		 * but we still need to initialize it to prevent crafted BPF
+		 * programs from reading them which would cause reading of
+		 * uninitialized data.
+		 */
+		*(__be16 *)skb_push(skb, 2) = htons(PPP_FILTER_OUTBOUND_TAG);
 		if (ppp->pass_filter &&
 		    bpf_prog_run(ppp->pass_filter, skb) == 0) {
 			if (ppp->debug & 1)
-- 
2.47.1