netdev - Re: [RFC] netlink: limit recursion depth in policy validation

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Date:   Fri, 26 Apr 2019 18:57:10 +0200
From:   Pablo Neira Ayuso <pablo@...filter.org>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     netdev@...r.kernel.org, Johannes Berg <johannes.berg@...el.com>
Subject: Re: [RFC] netlink: limit recursion depth in policy validation

On Fri, Apr 05, 2019 at 11:24:14PM +0200, Johannes Berg wrote:
> From: Johannes Berg <johannes.berg@...el.com>
> 
> Now that we have nested policies, we can theoretically
> recurse forever parsing attributes if a (sub-)policy
> refers back to a higher level one. This is a situation
> that has happened in nl80211, and we've avoided it there
> by not linking it.
> 
> Add some code to netlink parsing to limit recursion depth,
> allowing us to safely change nl80211 to actually link the
> nested policy, which in turn allows some code cleanups.
> 
> Signed-off-by: Johannes Berg <johannes.berg@...el.com>
> ---
>  lib/nlattr.c           | 46 +++++++++++++++++++++++++++++++-----------
>  net/wireless/nl80211.c | 10 ++++-----
>  net/wireless/nl80211.h |  2 --
>  net/wireless/pmsr.c    |  3 +--
>  4 files changed, 39 insertions(+), 22 deletions(-)
> 
> diff --git a/lib/nlattr.c b/lib/nlattr.c
> index baf27844ecc8..bc41d3d96945 100644
> --- a/lib/nlattr.c
> +++ b/lib/nlattr.c
> @@ -44,6 +44,20 @@ static const u8 nla_attr_minlen[NLA_TYPE_MAX+1] = {
>  	[NLA_S64]	= sizeof(s64),
>  };
>  
> +/*
> + * Nested policies might refer back to the original
> + * policy in some cases, and userspace could try to
> + * abuse that and recurse by nesting in the right
> + * ways. Limit recursion to avoid this problem.
> + */
> +#define MAX_POLICY_RECURSION_DEPTH	10

In your policy description approach, you iterate over the policy
structures. How do you deal with this recursions from there?