ghsa-934r-h8mp-qw4r
Vulnerability from github
Published
2024-05-17 12:31
Modified
2024-05-17 12:31
Details

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 won't 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 doesn't allow use of stateful nat, so we can safely discard the nf_conn entry and let inet call conntrack again.

This doesn't 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.

Show details on source website

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{
  "affected": [],
  "aliases": [
    "CVE-2024-27415"
  ],
  "database_specific": {
    "cwe_ids": [],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2024-05-17T12:15:12Z",
    "severity": null
  },
  "details": "In the Linux kernel, the following vulnerability has been resolved:\n\nnetfilter: bridge: confirm multicast packets before passing them up the stack\n\nconntrack nf_confirm logic cannot handle cloned skbs referencing\nthe same nf_conn entry, which will happen for multicast (broadcast)\nframes on bridges.\n\n Example:\n    macvlan0\n       |\n      br0\n     /  \\\n  ethX    ethY\n\n ethX (or Y) receives a L2 multicast or broadcast packet containing\n an IP packet, flow is not yet in conntrack table.\n\n 1. skb passes through bridge and fake-ip (br_netfilter)Prerouting.\n    -\u003e skb-\u003e_nfct now references a unconfirmed entry\n 2. skb is broad/mcast packet. bridge now passes clones out on each bridge\n    interface.\n 3. skb gets passed up the stack.\n 4. In macvlan case, macvlan driver retains clone(s) of the mcast skb\n    and schedules a work queue to send them out on the lower devices.\n\n    The clone skb-\u003e_nfct is not a copy, it is the same entry as the\n    original skb.  The macvlan rx handler then returns RX_HANDLER_PASS.\n 5. Normal conntrack hooks (in NF_INET_LOCAL_IN) confirm the orig skb.\n\nThe Macvlan broadcast worker and normal confirm path will race.\n\nThis race will not happen if step 2 already confirmed a clone. In that\ncase later steps perform skb_clone() with skb-\u003e_nfct already confirmed (in\nhash table).  This works fine.\n\nBut such confirmation won\u0027t happen when eb/ip/nftables rules dropped the\npackets before they reached the nf_confirm step in postrouting.\n\nPablo points out that nf_conntrack_bridge doesn\u0027t allow use of stateful\nnat, so we can safely discard the nf_conn entry and let inet call\nconntrack again.\n\nThis doesn\u0027t work for bridge netfilter: skb could have a nat\ntransformation. Also bridge nf prevents re-invocation of inet prerouting\nvia \u0027sabotage_in\u0027 hook.\n\nWork around this problem by explicit confirmation of the entry at LOCAL_IN\ntime, before upper layer has a chance to clone the unconfirmed entry.\n\nThe downside is that this disables NAT and conntrack helpers.\n\nAlternative fix would be to add locking to all code parts that deal with\nunconfirmed packets, but even if that could be done in a sane way this\nopens up other problems, for example:\n\n-m physdev --physdev-out eth0 -j SNAT --snat-to 1.2.3.4\n-m physdev --physdev-out eth1 -j SNAT --snat-to 1.2.3.5\n\nFor multicast case, only one of such conflicting mappings will be\ncreated, conntrack only handles 1:1 NAT mappings.\n\nUsers should set create a setup that explicitly marks such traffic\nNOTRACK (conntrack bypass) to avoid this, but we cannot auto-bypass\nthem, ruleset might have accept rules for untracked traffic already,\nso user-visible behaviour would change.",
  "id": "GHSA-934r-h8mp-qw4r",
  "modified": "2024-05-17T12:31:00Z",
  "published": "2024-05-17T12:31:00Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2024-27415"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/2b1414d5e94e477edff1d2c79030f1d742625ea0"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/62e7151ae3eb465e0ab52a20c941ff33bb6332e9"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/7c3f28599652acf431a2211168de4a583f30b6d5"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/80cd0487f630b5382734997c3e5e3003a77db315"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/cb734975b0ffa688ff6cc0eed463865bf07b6c01"
    }
  ],
  "schema_version": "1.4.0",
  "severity": []
}


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