linux-kernel - Re: [PATCH net 1/2] ioam6: fix write to cloned skb in ipv6_hop

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Date: Mon, 19 Feb 2024 10:05:03 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Justin Iurman <justin.iurman@...ege.be>, netdev@...r.kernel.org
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
 shuah@...nel.org,  linux-kselftest@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH net 1/2] ioam6: fix write to cloned skb in
 ipv6_hop_ioam()

On Sat, 2024-02-17 at 00:43 +0100, Justin Iurman wrote:
> ioam6_fill_trace_data() writes inside the skb payload without ensuring
> it's writeable (e.g., not cloned). This function is called both from the
> input and output path. The output path (ioam6_iptunnel) already does the
> check. This commit provides a fix for the input path, inside
> ipv6_hop_ioam().
> 
> Fixes: 9ee11f0fff20 ("ipv6: ioam: Data plane support for Pre-allocated Trace ")
> Reported-by: Paolo Abeni <pabeni@...hat.com>
> Signed-off-by: Justin Iurman <justin.iurman@...ege.be>
> ---
>  net/ipv6/exthdrs.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/net/ipv6/exthdrs.c b/net/ipv6/exthdrs.c
> index 4952ae792450..f68e5faab3aa 100644
> --- a/net/ipv6/exthdrs.c
> +++ b/net/ipv6/exthdrs.c
> @@ -943,6 +943,14 @@ static bool ipv6_hop_ioam(struct sk_buff *skb, int optoff)
>  		if (!skb_valid_dst(skb))
>  			ip6_route_input(skb);
>  
> +		if (skb_cloned(skb)) {
> +			if (pskb_expand_head(skb, 0, 0, GFP_ATOMIC))
> +				goto drop;

My personal preference would be for using skb_ensure_writable() here,
with write_len == optoff + hdr->opt_len.

> +
> +			hdr = (struct ioam6_hdr *)(skb_network_header(skb) + optoff);
> +			trace = (struct ioam6_trace_hdr *)((u8 *)hdr + sizeof(*hdr));

Note that this can potentially change the network header ptr and the
caller - ip6_parse_tlv() - has cached such value in 'nh'. You also need
to update ip6_parse_tlv() to reload such pointer.

Side note: a bunch of self-tests are apparently stuck after this
series. I think it's an unrelated problem. I'll try to have a better
look.

Cheers,

Paolo