In the Linux kernel, the following vulnerability has been resolved:

netfilter: nf_tables: don't fail inserts if duplicate has expired

nftables selftests fail:
run-tests.sh testcases/sets/0044interval_overlap_0
Expected: 0-2 . 0-3, got:
W: [FAILED]     ./testcases/sets/0044interval_overlap_0: got 1

Insertion must ignore duplicate but expired entries.

Moreover, there is a strange asymmetry in nft_pipapo_activate:

It refetches the current element, whereas the other ->activate callbacks
(bitmap, hash, rhash, rbtree) use elem->priv.
Same for .remove: other set implementations take elem->priv,
nft_pipapo_remove fetches elem->priv, then does a relookup,
remove this.

I suspect this was the reason for the change that prompted the
removal of the expired check in pipapo_get() in the first place,
but skipping exired elements there makes no sense to me, this helper
is used for normal get requests, insertions (duplicate check)
and deactivate callback.

In first two cases expired elements must be skipped.

For ->deactivate(), this gets called for DELSETELEM, so it
seems to me that expired elements should be skipped as well, i.e.
delete request should fail with -ENOENT error.

https://git.kernel.org/stable/c/156369a702c33ad5434a19c3a689bfb836d4e0b8

https://git.kernel.org/stable/c/59ee68c437c562170265194a99698c805a686bb3

https://git.kernel.org/stable/c/7845914f45f066497ac75b30c50dbc735e84e884

https://git.kernel.org/stable/c/891ca5dfe3b718b441fc786014a7ba8f517da188

https://git.kernel.org/stable/c/af78b0489e8898a8c9449ffc0fdd2e181916f0d4