Re: [PATCH] net: sched: fix possible OOB write in fl_set_geneve_opt()

[Simon Horman]

[Updated Pieter's email address, dropped old email address of mine]

On Mon, May 29, 2023 at 12:36:15PM +0800, Hangyu Hua wrote:
> If we send two TCA_FLOWER_KEY_ENC_OPTS_GENEVE packets and their total
> size is 252 bytes(key->enc_opts.len = 252) then
> key->enc_opts.len = opt->length = data_len / 4 = 0 when the third
> TCA_FLOWER_KEY_ENC_OPTS_GENEVE packet enters fl_set_geneve_opt. This
> bypasses the next bounds check and results in an out-of-bounds.
> 
> Fixes: 0a6e77784f49 ("net/sched: allow flower to match tunnel options")
> Signed-off-by: Hangyu Hua <hbh25y@xxxxxxxxx>

Hi Hangyu Hua,

Thanks. I think I see the problem too.
But I do wonder, is this more general than Geneve options?
That is, can this occur with any sequence of options, that
consume space in enc_opts (configured in fl_set_key()) that
in total are more than 256 bytes?

> ---
>  net/sched/cls_flower.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/net/sched/cls_flower.c b/net/sched/cls_flower.c
> index e960a46b0520..a326fbfe4339 100644
> --- a/net/sched/cls_flower.c
> +++ b/net/sched/cls_flower.c
> @@ -1153,6 +1153,9 @@ static int fl_set_geneve_opt(const struct nlattr *nla, struct fl_flow_key *key,
>  	if (option_len > sizeof(struct geneve_opt))
>  		data_len = option_len - sizeof(struct geneve_opt);
>  
> +	if (key->enc_opts.len > FLOW_DIS_TUN_OPTS_MAX - 4)
> +		return -ERANGE;
> +
>  	opt = (struct geneve_opt *)&key->enc_opts.data[key->enc_opts.len];
>  	memset(opt, 0xff, option_len);
>  	opt->length = data_len / 4;
> -- 
> 2.34.1
> 
>