netdev - Re: [Patch net v2] cls_flower: call nla_ok() before nla

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Date:   Thu, 14 Jan 2021 10:38:48 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     netdev@...r.kernel.org, Cong Wang <cong.wang@...edance.com>,
        syzbot+2624e3778b18fc497c92@...kaller.appspotmail.com,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Xin Long <lucien.xin@...il.com>, Jiri Pirko <jiri@...nulli.us>
Subject: Re: [Patch net v2] cls_flower: call nla_ok() before nla_next()

On Thu, 14 Jan 2021 08:38:22 -0800 Cong Wang wrote:
> From: Cong Wang <cong.wang@...edance.com>
> 
> fl_set_enc_opt() simply checks if there are still bytes left to parse,
> but this is not sufficent as syzbot seems to be able to generate
> malformatted netlink messages. nla_ok() is more strict so should be
> used to validate the next nlattr here.
> 
> And nla_validate_nested_deprecated() has less strict check too, it is
> probably too late to switch to the strict version, but we can just
> call nla_ok() too after it.
> 
> Reported-and-tested-by: syzbot+2624e3778b18fc497c92@...kaller.appspotmail.com
> Fixes: 0a6e77784f49 ("net/sched: allow flower to match tunnel options")
> Fixes: 79b1011cb33d ("net: sched: allow flower to match erspan options")

> @@ -1340,9 +1341,6 @@ static int fl_set_enc_opt(struct nlattr **tb, struct fl_flow_key *key,
>  				NL_SET_ERR_MSG(extack, "Key and mask miss aligned");
>  				return -EINVAL;
>  			}
> -
> -			if (msk_depth)
> -				nla_opt_msk = nla_next(nla_opt_msk, &msk_depth);
>  			break;
>  		case TCA_FLOWER_KEY_ENC_OPTS_ERSPAN:
>  			if (key->enc_opts.dst_opt_type) {
> @@ -1373,14 +1371,17 @@ static int fl_set_enc_opt(struct nlattr **tb, struct fl_flow_key *key,
>  				NL_SET_ERR_MSG(extack, "Key and mask miss aligned");
>  				return -EINVAL;
>  			}
> -
> -			if (msk_depth)
> -				nla_opt_msk = nla_next(nla_opt_msk, &msk_depth);
>  			break;
>  		default:
>  			NL_SET_ERR_MSG(extack, "Unknown tunnel option type");
>  			return -EINVAL;
>  		}
> +
> +		if (!nla_ok(nla_opt_msk, msk_depth)) {
> +			NL_SET_ERR_MSG(extack, "Mask attribute is invalid");
> +			return -EINVAL;
> +		}
> +		nla_opt_msk = nla_next(nla_opt_msk, &msk_depth);

we lost the if (msk_depth) now, nla_opt_msk may be NULL -
neither nla_ok() nor nla_next() take NULL

>  	}
>  
>  	return 0;