While checking the validity of insertion in __nft_rbtree_insert(),
we currently ignore conflicting elements and intervals only if they
are not active within the next generation.

However, if we consider expired elements and intervals as
potentially conflicting and overlapping, we'll return error for
entries that should be added instead. This is particularly visible
with garbage collection intervals that are comparable with the
element timeout itself, as reported by Mike Dillinger.

Other than the simple issue of denying insertion of valid entries,
this might also result in insertion of a single element (opening or
closing) out of a given interval. With single entries (that are
inserted as intervals of size 1), this leads in turn to the creation
of new intervals. For example:

  # nft add element t s { 192.0.2.1 }
  # nft list ruleset
  [...]
     elements = { 192.0.2.1-255.255.255.255 }

Always ignore expired elements active in the next generation, while
checking for conflicts.

It might be more convenient to introduce a new macro that covers
both inactive and expired items, as this type of check also appears
quite frequently in other set back-ends. This is however beyond the
scope of this fix and can be deferred to a separate patch.

Other than the overlap detection cases introduced by commit
7c84d414

 ("netfilter: nft_set_rbtree: Detect partial overlaps
on insertion"), we also have to cover the original conflict check
dealing with conflicts between two intervals of size 1, which was
introduced before support for timeout was introduced. This won't
return an error to the user as -EEXIST is masked by nft if
NLM_F_EXCL is not given, but would result in a silent failure
adding the entry.

Reported-by: Mike Dillinger <miked@softtalker.com>
Cc: <stable@vger.kernel.org> # 5.6.x
Fixes: 8d8540c4 ("netfilter: nft_set_rbtree: add timeout support")
Fixes: 7c84d414

 ("netfilter: nft_set_rbtree: Detect partial overlaps on insertion")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Acked-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>