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

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Message-ID: <7be848796a1f5552e611e131c8613408c128bfc8.camel@sipsolutions.net>
Date:   Fri, 26 Apr 2019 19:03:10 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Pablo Neira Ayuso <pablo@...filter.org>
Cc:     netdev@...r.kernel.org
Subject: Re: [RFC] netlink: limit recursion depth in policy validation

On Fri, 2019-04-26 at 18:57 +0200, Pablo Neira Ayuso wrote:
> 
> > +/*
> > + * 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?

Well, check out the code :-)

It doesn't actually recurse. What it does is build a list of policies
that are reachable from the root policy and each policy in the list. So
basically, there we do:

list = [root policy]
list_len = 1
i = 0

walk_policy(policy)
{
   for_each_policy_entry(entry, policy) {
      nested = nested_policy_or_null(entry);
      if (nested) {
         list[i] = nested;
         list_len += 1
      }
   }
}

while (i < list_len) {
    walk_policy(list[i]);
    i++;
}

Then, we walk the list again:

for (i = 0; i < list_len; i++) {
    for_each_policy_entry(entry, list[i]) {
       send_entry_to_userspace(i, entry); // mark it as occurring in policy i
    }
}


This basically flattens the whole thing.

Obviously, the walking may allocate some memory, and the last loop to
send it out isn't actually a loop like that because it's a netlink dump
with each entry being in a separate netlink message, but that's the gist
of it.

johannes