{"api_version":"1","generated_at":"2026-07-24T18:27:14+00:00","cve":"CVE-2025-21702","urls":{"html":"https://cve.report/CVE-2025-21702","api":"https://cve.report/api/cve/CVE-2025-21702.json","docs":"https://cve.report/api","cve_org":"https://www.cve.org/CVERecord?id=CVE-2025-21702","nvd":"https://nvd.nist.gov/vuln/detail/CVE-2025-21702"},"summary":{"title":"pfifo_tail_enqueue: Drop new packet when sch->limit == 0","description":"In the Linux kernel, the following vulnerability has been resolved:\n\npfifo_tail_enqueue: Drop new packet when sch->limit == 0\n\nExpected behaviour:\nIn case we reach scheduler's limit, pfifo_tail_enqueue() will drop a\npacket in scheduler's queue and decrease scheduler's qlen by one.\nThen, pfifo_tail_enqueue() enqueue new packet and increase\nscheduler's qlen by one. Finally, pfifo_tail_enqueue() return\n`NET_XMIT_CN` status code.\n\nWeird behaviour:\nIn case we set `sch->limit == 0` and trigger pfifo_tail_enqueue() on a\nscheduler that has no packet, the 'drop a packet' step will do nothing.\nThis means the scheduler's qlen still has value equal 0.\nThen, we continue to enqueue new packet and increase scheduler's qlen by\none. In summary, we can leverage pfifo_tail_enqueue() to increase qlen by\none and return `NET_XMIT_CN` status code.\n\nThe problem is:\nLet's say we have two qdiscs: Qdisc_A and Qdisc_B.\n - Qdisc_A's type must have '->graft()' function to create parent/child relationship.\n   Let's say Qdisc_A's type is `hfsc`. Enqueue packet to this qdisc will trigger `hfsc_enqueue`.\n - Qdisc_B's type is pfifo_head_drop. Enqueue packet to this qdisc will trigger `pfifo_tail_enqueue`.\n - Qdisc_B is configured to have `sch->limit == 0`.\n - Qdisc_A is configured to route the enqueued's packet to Qdisc_B.\n\nEnqueue packet through Qdisc_A will lead to:\n - hfsc_enqueue(Qdisc_A) -> pfifo_tail_enqueue(Qdisc_B)\n - Qdisc_B->q.qlen += 1\n - pfifo_tail_enqueue() return `NET_XMIT_CN`\n - hfsc_enqueue() check for `NET_XMIT_SUCCESS` and see `NET_XMIT_CN` => hfsc_enqueue() don't increase qlen of Qdisc_A.\n\nThe whole process lead to a situation where Qdisc_A->q.qlen == 0 and Qdisc_B->q.qlen == 1.\nReplace 'hfsc' with other type (for example: 'drr') still lead to the same problem.\nThis violate the design where parent's qlen should equal to the sum of its childrens'qlen.\n\nBug impact: This issue can be used for user->kernel privilege escalation when it is reachable.","state":"PUBLISHED","assigner":"Linux","published_at":"2025-02-18 15:15:18","updated_at":"2026-07-14 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the Linux kernel, the following vulnerability has been resolved:\n\npfifo_tail_enqueue: Drop new packet when sch->limit == 0\n\nExpected behaviour:\nIn case we reach scheduler's limit, pfifo_tail_enqueue() will drop a\npacket in scheduler's queue and decrease scheduler's qlen by one.\nThen, pfifo_tail_enqueue() enqueue new packet and increase\nscheduler's qlen by one. Finally, pfifo_tail_enqueue() return\n`NET_XMIT_CN` status code.\n\nWeird behaviour:\nIn case we set `sch->limit == 0` and trigger pfifo_tail_enqueue() on a\nscheduler that has no packet, the 'drop a packet' step will do nothing.\nThis means the scheduler's qlen still has value equal 0.\nThen, we continue to enqueue new packet and increase scheduler's qlen by\none. In summary, we can leverage pfifo_tail_enqueue() to increase qlen by\none and return `NET_XMIT_CN` status code.\n\nThe problem is:\nLet's say we have two qdiscs: Qdisc_A and Qdisc_B.\n - Qdisc_A's type must have '->graft()' function to create parent/child relationship.\n   Let's say Qdisc_A's type is `hfsc`. Enqueue packet to this qdisc will trigger `hfsc_enqueue`.\n - Qdisc_B's type is pfifo_head_drop. Enqueue packet to this qdisc will trigger `pfifo_tail_enqueue`.\n - Qdisc_B is configured to have `sch->limit == 0`.\n - Qdisc_A is configured to route the enqueued's packet to Qdisc_B.\n\nEnqueue packet through Qdisc_A will lead to:\n - hfsc_enqueue(Qdisc_A) -> pfifo_tail_enqueue(Qdisc_B)\n - Qdisc_B->q.qlen += 1\n - pfifo_tail_enqueue() return `NET_XMIT_CN`\n - hfsc_enqueue() check for `NET_XMIT_SUCCESS` and see `NET_XMIT_CN` => hfsc_enqueue() don't increase qlen of Qdisc_A.\n\nThe whole process lead to a situation where Qdisc_A->q.qlen == 0 and Qdisc_B->q.qlen == 1.\nReplace 'hfsc' with other type (for example: 'drr') still lead to the same problem.\nThis violate the design where parent's qlen should equal to the sum of its childrens'qlen.\n\nBug impact: This issue can be used for user->kernel privilege escalation when it is reachable."}],"metrics":[{"cvssV3_1":{"baseScore":7.8,"baseSeverity":"HIGH","vectorString":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H","version":"3.1"}}],"providerMetadata":{"dateUpdated":"2026-05-11T21:04:45.314Z","orgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","shortName":"Linux"},"references":[{"url":"https://git.kernel.org/stable/c/78285b53266d6d51fa4ff504a23df03852eba84e"},{"url":"https://git.kernel.org/stable/c/7a9723ec27aff5674f1fd4934608937f1d650980"},{"url":"https://git.kernel.org/stable/c/a56a6e8589a9b98d8171611fbcc1e45a15fd2455"},{"url":"https://git.kernel.org/stable/c/020ecb76812a0526f4130ab5aeb6dc7c773e7ab9"},{"url":"https://git.kernel.org/stable/c/79a955ea4a2e5ddf4a36328959de0de496419888"},{"url":"https://git.kernel.org/stable/c/e40cb34b7f247fe2e366fd192700d1b4f38196ca"},{"url":"https://git.kernel.org/stable/c/b6a079c3b6f95378f26e2aeda520cb3176f7067b"},{"url":"https://git.kernel.org/stable/c/647cef20e649c576dff271e018d5d15d998b629d"}],"title":"pfifo_tail_enqueue: Drop 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Drop new packet when sch->limit == 0","CVE":"CVE-2025-21702","Year":"2025"},"notes":[{"CveYear":"2025","CveId":"21702","Ordinal":"1","NoteData":"In the Linux kernel, the following vulnerability has been resolved:\n\npfifo_tail_enqueue: Drop new packet when sch->limit == 0\n\nExpected behaviour:\nIn case we reach scheduler's limit, pfifo_tail_enqueue() will drop a\npacket in scheduler's queue and decrease scheduler's qlen by one.\nThen, pfifo_tail_enqueue() enqueue new packet and increase\nscheduler's qlen by one. Finally, pfifo_tail_enqueue() return\n`NET_XMIT_CN` status code.\n\nWeird behaviour:\nIn case we set `sch->limit == 0` and trigger pfifo_tail_enqueue() on a\nscheduler that has no packet, the 'drop a packet' step will do nothing.\nThis means the scheduler's qlen still has value equal 0.\nThen, we continue to enqueue new packet and increase scheduler's qlen by\none. In summary, we can leverage pfifo_tail_enqueue() to increase qlen by\none and return `NET_XMIT_CN` status code.\n\nThe problem is:\nLet's say we have two qdiscs: Qdisc_A and Qdisc_B.\n - Qdisc_A's type must have '->graft()' function to create parent/child relationship.\n   Let's say Qdisc_A's type is `hfsc`. Enqueue packet to this qdisc will trigger `hfsc_enqueue`.\n - Qdisc_B's type is pfifo_head_drop. Enqueue packet to this qdisc will trigger `pfifo_tail_enqueue`.\n - Qdisc_B is configured to have `sch->limit == 0`.\n - Qdisc_A is configured to route the enqueued's packet to Qdisc_B.\n\nEnqueue packet through Qdisc_A will lead to:\n - hfsc_enqueue(Qdisc_A) -> pfifo_tail_enqueue(Qdisc_B)\n - Qdisc_B->q.qlen += 1\n - pfifo_tail_enqueue() return `NET_XMIT_CN`\n - hfsc_enqueue() check for `NET_XMIT_SUCCESS` and see `NET_XMIT_CN` => hfsc_enqueue() don't increase qlen of Qdisc_A.\n\nThe whole process lead to a situation where Qdisc_A->q.qlen == 0 and Qdisc_B->q.qlen == 1.\nReplace 'hfsc' with other type (for example: 'drr') still lead to the same problem.\nThis violate the design where parent's qlen should equal to the sum of its childrens'qlen.\n\nBug impact: This issue can be used for user->kernel privilege escalation when it is reachable.","Type":"Description","Title":"pfifo_tail_enqueue: Drop new packet when sch->limit == 0"}]}}}