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

[Jiayuan Chen]

On Mon, Feb 24, 2025 at 02:26:44PM -0800, Jakub Kicinski wrote:
> On Fri, 21 Feb 2025 14:12:19 +0800 Jiayuan Chen wrote:
> > +		/* 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);
> 
> The constant from the RFC deserves a #define or enum.
> Looks like we may already need it in one other place:
> 
> drivers/net/wan/fsl_ucc_hdlc.h:#define DEFAULT_PPP_HEAD    0xff03
> -- 
> pw-bot: cr
Hi Jakub,

I apologize for the mistake, I've investigated the original maintainer's
user-space PPP implementation and libpcap's behavior, and found that
initializing the first byte to 0 or 1 is necessary, it indicates
direction, which is used in libpcap to distinguish between inbound and
outbound traffic.
For more details, please refer to the cover letter of my v3 patch.

https://lore.kernel.org/linux-ppp/20250222092556.274267-1-jiayuan.chen@xxxxxxxxx/T/#t