[PATCH net-next v2 1/1] ppp: Fix KMSAN warning by initializing 2-byte header

[Jiayuan Chen <jiayuan.chen@linux.dev>]

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

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

Reported-by: syzbot+853242d9c9917165d791@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/bpf/000000000000dea025060d6bc3bc@google.com/
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 drivers/net/ppp/ppp_generic.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index 4583e15ad03a..ac95196c85df 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -1762,10 +1762,16 @@ 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.
+		 * BPF filter instructions assume each PPP packet has a 4-byte
+		 * header (e.g., those generated by libpcap), and then default
+		 * to skipping the first 2 bytes at the beginning of the
+		 * instruction. However, we still need to initialize these
+		 * 2-byte new headers to prevent crafted BPF programs from
+		 * reading them which would cause reading of uninitialized
+		 * data. Here, we set the headers according to the RFC 1662.
+		 */
+		*(u16 *)skb_push(skb, 2) = htons(0xff03);
 		if (ppp->pass_filter &&
 		    bpf_prog_run(ppp->pass_filter, skb) == 0) {
 			if (ppp->debug & 1)
-- 
2.47.1