[PATCH net-next v3 1/1] ppp: Fix KMSAN warning by initializing 2-byte header
From: Jiayuan Chen
Date: Sat Feb 22 2025 - 04:26:41 EST
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@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/bpf/000000000000dea025060d6bc3bc@xxxxxxxxxx/
Signed-off-by: Jiayuan Chen <jiayuan.chen@xxxxxxxxx>
---
drivers/net/ppp/ppp_generic.c | 15 +++++++++++----
1 file changed, 11 insertions(+), 4 deletions(-)
diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index 4583e15ad03a..29a7a21cb096 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -1762,10 +1762,17 @@ 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.
+ */
+ skb_push(skb, 2);
+ skb->data[0] = 1;
+ skb->data[1] = 0;
if (ppp->pass_filter &&
bpf_prog_run(ppp->pass_filter, skb) == 0) {
if (ppp->debug & 1)
--
2.47.1