{"id":"CVE-2025-21702","title":"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…","summary":"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…","severity":"high","cvss":7.8,"cvssVector":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H","vendor":"linux","product":"linux_kernel","affected":["linux_kernel >= 2.6.34, < 5.4.291","linux_kernel >= 5.5, < 5.10.235","linux_kernel >= 5.11, < 5.15.179","linux_kernel >= 5.16, < 6.1.130","linux_kernel >= 6.2, < 6.6.83","linux_kernel >= 6.7, < 6.12.14","linux_kernel >= 6.13, < 6.13.3","linux_kernel = 6.14"],"patched":["linux_kernel 6.13.3"],"published":"2025-02-18","updated":"2026-07-14","source":"NVD","sourceUrl":"https://nvd.nist.gov/vuln/detail/CVE-2025-21702","references":[{"url":"https://git.kernel.org/stable/c/020ecb76812a0526f4130ab5aeb6dc7c773e7ab9","label":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/647cef20e649c576dff271e018d5d15d998b629d","label":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/78285b53266d6d51fa4ff504a23df03852eba84e","label":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/79a955ea4a2e5ddf4a36328959de0de496419888","label":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/7a9723ec27aff5674f1fd4934608937f1d650980","label":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/a56a6e8589a9b98d8171611fbcc1e45a15fd2455","label":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/b6a079c3b6f95378f26e2aeda520cb3176f7067b","label":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/e40cb34b7f247fe2e366fd192700d1b4f38196ca","label":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://lists.debian.org/debian-lts-announce/2025/05/msg00030.html","label":"af854a3a-2127-422b-91ae-364da2661108"},{"url":"https://lists.debian.org/debian-lts-announce/2025/05/msg00045.html","label":"af854a3a-2127-422b-91ae-364da2661108"},{"url":"https://cert-portal.siemens.com/productcert/html/ssa-019113.html","label":"0b142b55-0307-4c5a-b3c9-f314f3fb7c5e"},{"url":"https://cert-portal.siemens.com/productcert/html/ssa-082556.html","label":"0b142b55-0307-4c5a-b3c9-f314f3fb7c5e"}],"tags":["nvd","cve.org"],"epss":0.00272,"epssPercentile":0.19823,"ingestedAt":"2026-07-14T13:36:54.977Z","slug":"CVE-2025-21702","body":"## Overview\n\nIn 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.\n\n## Affected\n\n- `linux_kernel >= 2.6.34, < 5.4.291`\n- `linux_kernel >= 5.5, < 5.10.235`\n- `linux_kernel >= 5.11, < 5.15.179`\n- `linux_kernel >= 5.16, < 6.1.130`\n- `linux_kernel >= 6.2, < 6.6.83`\n- `linux_kernel >= 6.7, < 6.12.14`\n- `linux_kernel >= 6.13, < 6.13.3`\n- `linux_kernel = 6.14`\n\n## Remediation\n\nUpgrade past the affected range:\n\n- `linux_kernel 6.13.3`","depth":"twilight","depthScore":43,"depthScoreParts":{"impact":42.9,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[]}