Re: [PATCH] net: ppp: Add bound checking for skb d on ppp_sync_txmung

[Simon Horman]

On Mon, Apr 07, 2025 at 05:26:21PM +0200, Arnaud Lecomte wrote:
> Ensure we have enough data in linear buffer from skb before accessing
> initial bytes. This prevents potential out-of-bounds accesses
> when processing short packets.
> 
> When ppp_sync_txmung receives an incoming package with an empty
> payload:
> (remote) gef➤  p *(struct pppoe_hdr *) (skb->head + skb->network_header)
> $18 = {
> 	type = 0x1,
> 	ver = 0x1,
> 	code = 0x0,
> 	sid = 0x2,
>         length = 0x0,
> 	tag = 0xffff8880371cdb96
> }
> 
> from the skb struct (trimmed)
>       tail = 0x16,
>       end = 0x140,
>       head = 0xffff88803346f400 "4",
>       data = 0xffff88803346f416 ":\377",
>       truesize = 0x380,
>       len = 0x0,
>       data_len = 0x0,
>       mac_len = 0xe,
>       hdr_len = 0x0,
> 
> it is not safe to access data[2].
> 
> Reported-by: syzbot+29fc8991b0ecb186cf40@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=29fc8991b0ecb186cf40
> Tested-by: syzbot+29fc8991b0ecb186cf40@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: 9946eaf552b1 ("Merge tag 'hardening-v6.14-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux")

It doesn't seem right to use a Merge commit in a fixes tag.

Looking over the code, the access to data[2] seems to have existed since
the beginning of git history, in which case I think we can use this:

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")

> Signed-off-by: Arnaud Lecomte <contact@xxxxxxxxxxxxxx>
> ---
>  drivers/net/ppp/ppp_synctty.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/net/ppp/ppp_synctty.c b/drivers/net/ppp/ppp_synctty.c
> index 644e99fc3623..520d895acc60 100644
> --- a/drivers/net/ppp/ppp_synctty.c
> +++ b/drivers/net/ppp/ppp_synctty.c
> @@ -506,6 +506,11 @@ ppp_sync_txmunge(struct syncppp *ap, struct sk_buff *skb)
>  	unsigned char *data;
>  	int islcp;
>  
> +	/* Ensure we can safely access protocol field and LCP code */
> +	if (!skb || !pskb_may_pull(skb, 3)) {

I doubt that skb can be NULL.

> +		kfree_skb(skb);
> +		return NULL;
> +	}
>  	data  = skb->data;
>  	proto = get_unaligned_be16(data);
>  
> 
> ---
> base-commit: 9946eaf552b194bb352c2945b54ff98c8193b3f1
> change-id: 20250405-bound-checking-ppp_txmung-4807c854ed85
> 
> Best regards,
> -- 
> Arnaud Lecomte <contact@xxxxxxxxxxxxxx>
> 
>