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 | 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..4019bc959a2a 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. + */ + *(u16 *)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
