netfilter: nft_set_pipapo: fix stack out-of-bounds read in pipapo_drop()

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

netfilter: nft_set_pipapo: fix stack out-of-bounds read in pipapo_drop()

pipapo_drop() passes rulemap[i + 1].n to pipapo_unmap() as the

to_offset argument on every iteration, including the last one where

i == m->field_count - 1. This reads one element past the end of the

stack-allocated rulemap array (declared as rulemap[NFT_PIPAPO_MAX_FIELDS]

with NFT_PIPAPO_MAX_FIELDS == 16).

Although pipapo_unmap() returns early when is_last is true without

using the to_offset value, the argument is evaluated at the call site

before the function body executes, making this a genuine out-of-bounds

stack read confirmed by KASAN:

BUG: KASAN: stack-out-of-bounds in pipapo_drop+0x50c/0x57c [nf_tables]

Read of size 4 at addr ffff8000810e71a4

This frame has 1 object:

[32, 160) 'rulemap'

The buggy address is at offset 164 -- exactly 4 bytes past the end

of the rulemap array.

Pass 0 instead of rulemap[i + 1].n on the last iteration to avoid

the out-of-bounds read.