@@ -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)
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(-)