In the Linux kernel, the following vulnerability has been resolved:
netfilter: nf_tables: don't skip expired elements during walk
There is an asymmetry between commit/abort and preparation phase if the following conditions are met:
- set is a verdict map ("1.2.3.4 : jump foo")
- timeouts are enabled
In this case, following sequence is problematic:
- element E in set S refers to chain C
- userspace requests removal of set S
- kernel does a set walk to decrement chain->use count for all elements
- kernel does another set walk to remove elements from the commit phase
If E has already expired in 1), it will be ignored during list walk, so its use count won't have been changed.
Then, when set is culled, ->destroy callback will zap the element via nf_tables_set_elem_destroy(), but this function is only safe for elements that have been deactivated earlier from the preparation phase: lack of earlier deactivate removes the element but leaks the chain use count, which results in a WARN splat when the chain gets removed later, plus a leak of the nft_chain structure.
Update pipapo_get() not to skip expired elements, otherwise flush command reports bogus ENOENT errors.