Re: [PATCH] ppp: Prevent out-of-bounds access in ppp_sync_txmunge

[Kuniyuki Iwashima]

From: Purva Yeshi <purvayeshi550@xxxxxxxxx>
Date: Sun, 16 Feb 2025 11:34:46 +0530
> Fix an issue detected by syzbot with KMSAN:
> 
> BUG: KMSAN: uninit-value in ppp_sync_txmunge
> drivers/net/ppp/ppp_synctty.c:516 [inline]
> BUG: KMSAN: uninit-value in ppp_sync_send+0x21c/0xb00
> drivers/net/ppp/ppp_synctty.c:568
> 
> Ensure sk_buff is valid and has at least 3 bytes before accessing its
> data field in ppp_sync_txmunge(). Without this check, the function may
> attempt to read uninitialized or invalid memory, leading to undefined
> behavior.
> 
> To address this, add a validation check at the beginning of the function
> to safely handle cases where skb is NULL or too small. If either condition
> is met, free the skb and return NULL to prevent processing an invalid
> packet.
> 
> Reported-by: syzbot+29fc8991b0ecb186cf40@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=29fc8991b0ecb186cf40
> Tested-by: syzbot+29fc8991b0ecb186cf40@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Purva Yeshi <purvayeshi550@xxxxxxxxx>
> ---
>  drivers/net/ppp/ppp_synctty.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/net/ppp/ppp_synctty.c b/drivers/net/ppp/ppp_synctty.c
> index 644e99fc3..e537ea3d9 100644
> --- a/drivers/net/ppp/ppp_synctty.c
> +++ b/drivers/net/ppp/ppp_synctty.c
> @@ -506,6 +506,12 @@ ppp_sync_txmunge(struct syncppp *ap, struct sk_buff *skb)
>  	unsigned char *data;
>  	int islcp;
>  
> +	/* Ensure skb is not NULL and has at least 3 bytes */
> +	if (!skb || skb->len < 3) {

When is skb NULL ?


> +		kfree_skb(skb);
> +		return NULL;
> +	}
> +
>  	data  = skb->data;
>  	proto = get_unaligned_be16(data);
>  
> -- 
> 2.34.1