CVE-2024-41042

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

netfilter: nftables: prefer nftchain_validate

nftchainvalidate already performs loop detection because a cycle will result in a call stack overflow (ctx->level >= NFTJUMPSTACK_SIZE).

It also follows maps via ->validate callback in nft_lookup, so there appears no reason to iterate the maps again.

nftablescheck_loops() and all its helper functions can be removed. This improves ruleset load time significantly, from 23s down to 12s.

This also fixes a crash bug. Old loop detection code can result in unbounded recursion:

BUG: TASK stack guard page was hit at .... Oops: stack guard page: 0000 [#1] PREEMPT SMP KASAN CPU: 4 PID: 1539 Comm: nft Not tainted 6.10.0-rc5+ #1 [..]

with a suitable ruleset during validation of register stores.

I can't see any actual reason to attempt to check for this from nftvalidateregister_store(), at this point the transaction is still in progress, so we don't have a full picture of the rule graph.

For nf-next it might make sense to either remove it or make this depend on table->validate_state in case we could catch an error earlier (for improved error reporting to userspace).