ghsa-m7p9-xw5x-w2c4
Vulnerability from github
In the Linux kernel, the following vulnerability has been resolved:
net/sched: netem: account for backlog updates from child qdisc
In general, 'qlen' of any classful qdisc should keep track of the number of packets that the qdisc itself and all of its children holds. In case of netem, 'qlen' only accounts for the packets in its internal tfifo. When netem is used with a child qdisc, the child qdisc can use 'qdisc_tree_reduce_backlog' to inform its parent, netem, about created or dropped SKBs. This function updates 'qlen' and the backlog statistics of netem, but netem does not account for changes made by a child qdisc. 'qlen' then indicates the wrong number of packets in the tfifo. If a child qdisc creates new SKBs during enqueue and informs its parent about this, netem's 'qlen' value is increased. When netem dequeues the newly created SKBs from the child, the 'qlen' in netem is not updated. If 'qlen' reaches the configured sch->limit, the enqueue function stops working, even though the tfifo is not full.
Reproduce the bug: Ensure that the sender machine has GSO enabled. Configure netem as root qdisc and tbf as its child on the outgoing interface of the machine as follows: $ tc qdisc add dev root handle 1: netem delay 100ms limit 100 $ tc qdisc add dev parent 1:0 tbf rate 50Mbit burst 1542 latency 50ms
Send bulk TCP traffic out via this interface, e.g., by running an iPerf3 client on the machine. Check the qdisc statistics: $ tc -s qdisc show dev
Statistics after 10s of iPerf3 TCP test before the fix (note that netem's backlog > limit, netem stopped accepting packets): qdisc netem 1: root refcnt 2 limit 1000 delay 100ms Sent 2767766 bytes 1848 pkt (dropped 652, overlimits 0 requeues 0) backlog 4294528236b 1155p requeues 0 qdisc tbf 10: parent 1:1 rate 50Mbit burst 1537b lat 50ms Sent 2767766 bytes 1848 pkt (dropped 327, overlimits 7601 requeues 0) backlog 0b 0p requeues 0
Statistics after the fix: qdisc netem 1: root refcnt 2 limit 1000 delay 100ms Sent 37766372 bytes 24974 pkt (dropped 9, overlimits 0 requeues 0) backlog 0b 0p requeues 0 qdisc tbf 10: parent 1:1 rate 50Mbit burst 1537b lat 50ms Sent 37766372 bytes 24974 pkt (dropped 327, overlimits 96017 requeues 0) backlog 0b 0p requeues 0
tbf segments the GSO SKBs (tbf_segment) and updates the netem's 'qlen'. The interface fully stops transferring packets and "locks". In this case, the child qdisc and tfifo are empty, but 'qlen' indicates the tfifo is at its limit and no more packets are accepted.
This patch adds a counter for the entries in the tfifo. Netem's 'qlen' is only decreased when a packet is returned by its dequeue function, and not during enqueuing into the child qdisc. External updates to 'qlen' are thus accounted for and only the behavior of the backlog statistics changes. As in other qdiscs, 'qlen' then keeps track of how many packets are held in netem and all of its children. As before, sch->limit remains as the maximum number of packets in the tfifo. The same applies to netem's backlog statistics.
{
"affected": [],
"aliases": [
"CVE-2024-56770"
],
"database_specific": {
"cwe_ids": [],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2025-01-08T17:15:15Z",
"severity": "MODERATE"
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\nnet/sched: netem: account for backlog updates from child qdisc\n\nIn general, \u0027qlen\u0027 of any classful qdisc should keep track of the\nnumber of packets that the qdisc itself and all of its children holds.\nIn case of netem, \u0027qlen\u0027 only accounts for the packets in its internal\ntfifo. When netem is used with a child qdisc, the child qdisc can use\n\u0027qdisc_tree_reduce_backlog\u0027 to inform its parent, netem, about created\nor dropped SKBs. This function updates \u0027qlen\u0027 and the backlog statistics\nof netem, but netem does not account for changes made by a child qdisc.\n\u0027qlen\u0027 then indicates the wrong number of packets in the tfifo.\nIf a child qdisc creates new SKBs during enqueue and informs its parent\nabout this, netem\u0027s \u0027qlen\u0027 value is increased. When netem dequeues the\nnewly created SKBs from the child, the \u0027qlen\u0027 in netem is not updated.\nIf \u0027qlen\u0027 reaches the configured sch-\u003elimit, the enqueue function stops\nworking, even though the tfifo is not full.\n\nReproduce the bug:\nEnsure that the sender machine has GSO enabled. Configure netem as root\nqdisc and tbf as its child on the outgoing interface of the machine\nas follows:\n$ tc qdisc add dev \u003coif\u003e root handle 1: netem delay 100ms limit 100\n$ tc qdisc add dev \u003coif\u003e parent 1:0 tbf rate 50Mbit burst 1542 latency 50ms\n\nSend bulk TCP traffic out via this interface, e.g., by running an iPerf3\nclient on the machine. Check the qdisc statistics:\n$ tc -s qdisc show dev \u003coif\u003e\n\nStatistics after 10s of iPerf3 TCP test before the fix (note that\nnetem\u0027s backlog \u003e limit, netem stopped accepting packets):\nqdisc netem 1: root refcnt 2 limit 1000 delay 100ms\n Sent 2767766 bytes 1848 pkt (dropped 652, overlimits 0 requeues 0)\n backlog 4294528236b 1155p requeues 0\nqdisc tbf 10: parent 1:1 rate 50Mbit burst 1537b lat 50ms\n Sent 2767766 bytes 1848 pkt (dropped 327, overlimits 7601 requeues 0)\n backlog 0b 0p requeues 0\n\nStatistics after the fix:\nqdisc netem 1: root refcnt 2 limit 1000 delay 100ms\n Sent 37766372 bytes 24974 pkt (dropped 9, overlimits 0 requeues 0)\n backlog 0b 0p requeues 0\nqdisc tbf 10: parent 1:1 rate 50Mbit burst 1537b lat 50ms\n Sent 37766372 bytes 24974 pkt (dropped 327, overlimits 96017 requeues 0)\n backlog 0b 0p requeues 0\n\ntbf segments the GSO SKBs (tbf_segment) and updates the netem\u0027s \u0027qlen\u0027.\nThe interface fully stops transferring packets and \"locks\". In this case,\nthe child qdisc and tfifo are empty, but \u0027qlen\u0027 indicates the tfifo is at\nits limit and no more packets are accepted.\n\nThis patch adds a counter for the entries in the tfifo. Netem\u0027s \u0027qlen\u0027 is\nonly decreased when a packet is returned by its dequeue function, and not\nduring enqueuing into the child qdisc. External updates to \u0027qlen\u0027 are thus\naccounted for and only the behavior of the backlog statistics changes. As\nin other qdiscs, \u0027qlen\u0027 then keeps track of how many packets are held in\nnetem and all of its children. As before, sch-\u003elimit remains as the\nmaximum number of packets in the tfifo. The same applies to netem\u0027s\nbacklog statistics.",
"id": "GHSA-m7p9-xw5x-w2c4",
"modified": "2025-01-10T18:31:39Z",
"published": "2025-01-08T18:30:48Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2024-56770"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/10df49cfca73dfbbdb6c4150d859f7e8926ae427"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/216509dda290f6db92c816dd54b83c1df9da9e76"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/356078a5c55ec8d2061fcc009fb8599f5b0527f9"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/3824c5fad18eeb7abe0c4fc966f29959552dca3e"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/83c6ab12f08dcc09d4c5ac86fdb89736b28f1d31"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/c2047b0e216c8edce227d7c42f99ac2877dad0e4"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/f8d4bc455047cf3903cd6f85f49978987dbb3027"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H",
"type": "CVSS_V3"
}
]
}
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