CVE-2024-27415

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

netfilter: bridge: confirm multicast packets before passing them up the stack

conntrack nf_confirm logic cannot handle cloned skbs referencing the same nf_conn entry, which will happen for multicast (broadcast) frames on bridges.

Example: macvlan0 | br0 /
ethX ethY

ethX (or Y) receives a L2 multicast or broadcast packet containing an IP packet, flow is not yet in conntrack table.

1. skb passes through bridge and fake-ip (br_netfilter)Prerouting. -> skb->_nfct now references a unconfirmed entry

2. skb is broad/mcast packet. bridge now passes clones out on each bridge interface.

3. skb gets passed up the stack.

4. In macvlan case, macvlan driver retains clone(s) of the mcast skb and schedules a work queue to send them out on the lower devices.

   The clone skb->_nfct is not a copy, it is the same entry as the original skb. The macvlan rx handler then returns RX_HANDLER_PASS.

5. Normal conntrack hooks (in NF_INET_LOCAL_IN) confirm the orig skb.

The Macvlan broadcast worker and normal confirm path will race.

This race will not happen if step 2 already confirmed a clone. In that case later steps perform skb_clone() with skb->_nfct already confirmed (in hash table). This works fine.

But such confirmation wont happen when eb/ip/nftables rules dropped the packets before they reached the nf_confirm step in postrouting.

Pablo points out that nf_conntrack_bridge doesnt allow use of stateful nat, so we can safely discard the nf_conn entry and let inet call conntrack again.

This doesnt work for bridge netfilter: skb could have a nat transformation. Also bridge nf prevents re-invocation of inet prerouting via sabotage_in hook.

Work around this problem by explicit confirmation of the entry at LOCAL_IN time, before upper layer has a chance to clone the unconfirmed entry.

The downside is that this disables NAT and conntrack helpers.

Alternative fix would be to add locking to all code parts that deal with unconfirmed packets, but even if that could be done in a sane way this opens up other problems, for example:

-m physdev –physdev-out eth0 -j SNAT –snat-to 1.2.3.4 -m physdev –physdev-out eth1 -j SNAT –snat-to 1.2.3.5

For multicast case, only one of such conflicting mappings will be created, conntrack only handles 1:1 NAT mappings.

Users should set create a setup that explicitly marks such traffic NOTRACK (conntrack bypass) to avoid this, but we cannot auto-bypass them, ruleset might have accept rules for untracked traffic already, so user-visible behaviour would change.

Weakness

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.

Affected Software

Extended Description

A race condition occurs within concurrent environments, and it is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc. A race condition violates these properties, which are closely related:

A race condition exists when an “interfering code sequence” can still access the shared resource, violating exclusivity. The interfering code sequence could be “trusted” or “untrusted.” A trusted interfering code sequence occurs within the product; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable product.

Potential Mitigations

• Minimize the usage of shared resources in order to remove as much complexity as possible from the control flow and to reduce the likelihood of unexpected conditions occurring.
• Additionally, this will minimize the amount of synchronization necessary and may even help to reduce the likelihood of a denial of service where an attacker may be able to repeatedly trigger a critical section (CWE-400).

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/26.html
• https://cwe.mitre.org/data/definitions/29.html

References

• https://git.kernel.org/stable/c/2b1414d5e94e477edff1d2c79030f1d742625ea0
• https://git.kernel.org/stable/c/62e7151ae3eb465e0ab52a20c941ff33bb6332e9
• https://git.kernel.org/stable/c/7c3f28599652acf431a2211168de4a583f30b6d5
• https://git.kernel.org/stable/c/80cd0487f630b5382734997c3e5e3003a77db315
• https://git.kernel.org/stable/c/cb734975b0ffa688ff6cc0eed463865bf07b6c01
• https://git.kernel.org/stable/c/2b1414d5e94e477edff1d2c79030f1d742625ea0
• https://git.kernel.org/stable/c/62e7151ae3eb465e0ab52a20c941ff33bb6332e9
• https://git.kernel.org/stable/c/7c3f28599652acf431a2211168de4a583f30b6d5
• https://git.kernel.org/stable/c/80cd0487f630b5382734997c3e5e3003a77db315
• https://git.kernel.org/stable/c/cb734975b0ffa688ff6cc0eed463865bf07b6c01