CVE-2024-26924

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

netfilter: nft_set_pipapo: do not free live element

Pablo reports a crash with large batches of elements with a

back-to-back add/remove pattern. Quoting Pablo:

add_elem("00000000") timeout 100 ms

...

add_elem("0000000X") timeout 100 ms

del_elem("0000000X") <---------------- delete one that was just added

...

add_elem("00005000") timeout 100 ms

1) nft_pipapo_remove() removes element 0000000X

Then, KASAN shows a splat.

Looking at the remove function there is a chance that we will drop a

rule that maps to a non-deactivated element.

Removal happens in two steps, first we do a lookup for key k and return the

to-be-removed element and mark it as inactive in the next generation.

Then, in a second step, the element gets removed from the set/map.

The _remove function does not work correctly if we have more than one

element that share the same key.

This can happen if we insert an element into a set when the set already

holds an element with same key, but the element mapping to the existing

key has timed out or is not active in the next generation.

In such case its possible that removal will unmap the wrong element.

If this happens, we will leak the non-deactivated element, it becomes

unreachable.

The element that got deactivated (and will be freed later) will

remain reachable in the set data structure, this can result in

a crash when such an element is retrieved during lookup (stale

pointer).

Add a check that the fully matching key does in fact map to the element

that we have marked as inactive in the deactivation step.

If not, we need to continue searching.

Add a bug/warn trap at the end of the function as well, the remove

function must not ever be called with an invisible/unreachable/non-existent

element.

v2: avoid uneeded temporary variable (Stefano)