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

Jiayuan Chen
2025-02-26T01:37:22Z

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@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 | 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