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

Kuniyuki Iwashima
2025-02-17T21:16:23Z

From: Purva Yeshi <purvayeshi550@gmail.com>
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@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=29fc8991b0ecb186cf40
> Tested-by: syzbot+29fc8991b0ecb186cf40@syzkaller.appspotmail.com
> Signed-off-by: Purva Yeshi <purvayeshi550@gmail.com>
> ---
> 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