Description

In the Linux kernel, the following vulnerability has been resolved: pfifo_tail_enqueue: Drop new packet when sch->limit == 0 Expected behaviour: In case we reach scheduler's limit, pfifo_tail_enqueue() will drop a packet in scheduler's queue and decrease scheduler's qlen by one. Then, pfifo_tail_enqueue() enqueue new packet and increase scheduler's qlen by one. Finally, pfifo_tail_enqueue() return `NET_XMIT_CN` status code. Weird behaviour: In case we set `sch->limit == 0` and trigger pfifo_tail_enqueue() on a scheduler that has no packet, the 'drop a packet' step will do nothing. This means the scheduler's qlen still has value equal 0. Then, we continue to enqueue new packet and increase scheduler's qlen by one. In summary, we can leverage pfifo_tail_enqueue() to increase qlen by one and return `NET_XMIT_CN` status code. The problem is: Let's say we have two qdiscs: Qdisc_A and Qdisc_B. - Qdisc_A's type must have '->graft()' function to create parent/child relationship. Let's say Qdisc_A's type is `hfsc`. Enqueue packet to this qdisc will trigger `hfsc_enqueue`. - Qdisc_B's type is pfifo_head_drop. Enqueue packet to this qdisc will trigger `pfifo_tail_enqueue`. - Qdisc_B is configured to have `sch->limit == 0`. - Qdisc_A is configured to route the enqueued's packet to Qdisc_B. Enqueue packet through Qdisc_A will lead to: - hfsc_enqueue(Qdisc_A) -> pfifo_tail_enqueue(Qdisc_B) - Qdisc_B->q.qlen += 1 - pfifo_tail_enqueue() return `NET_XMIT_CN` - hfsc_enqueue() check for `NET_XMIT_SUCCESS` and see `NET_XMIT_CN` => hfsc_enqueue() don't increase qlen of Qdisc_A. The whole process lead to a situation where Qdisc_A->q.qlen == 0 and Qdisc_B->q.qlen == 1. Replace 'hfsc' with other type (for example: 'drr') still lead to the same problem. This violate the design where parent's qlen should equal to the sum of its childrens'qlen. Bug impact: This issue can be used for user->kernel privilege escalation when it is reachable.

Affected Products 8

References 11

• cert-portal.siemens.com https://cert-portal.siemens.com/productcert/html/ssa-082556.html
• git.kernel.org https://git.kernel.org/stable/c/020ecb76812a0526f4130ab5aeb6dc7c773e7ab9
  Patch
• git.kernel.org https://git.kernel.org/stable/c/647cef20e649c576dff271e018d5d15d998b629d
  Patch
• git.kernel.org https://git.kernel.org/stable/c/78285b53266d6d51fa4ff504a23df03852eba84e
  Patch
• git.kernel.org https://git.kernel.org/stable/c/79a955ea4a2e5ddf4a36328959de0de496419888
  Patch
• git.kernel.org https://git.kernel.org/stable/c/7a9723ec27aff5674f1fd4934608937f1d650980
  Patch
• git.kernel.org https://git.kernel.org/stable/c/a56a6e8589a9b98d8171611fbcc1e45a15fd2455
  Patch
• git.kernel.org https://git.kernel.org/stable/c/b6a079c3b6f95378f26e2aeda520cb3176f7067b
  Patch
• git.kernel.org https://git.kernel.org/stable/c/e40cb34b7f247fe2e366fd192700d1b4f38196ca
  Patch
• lists.debian.org https://lists.debian.org/debian-lts-announce/2025/05/msg00030.html
• lists.debian.org https://lists.debian.org/debian-lts-announce/2025/05/msg00045.html

Remediation

• git.kernel.org https://git.kernel.org/stable/c/020ecb76812a0526f4130ab5aeb6dc7c773e7ab9
  Patch
• git.kernel.org https://git.kernel.org/stable/c/647cef20e649c576dff271e018d5d15d998b629d
  Patch
• git.kernel.org https://git.kernel.org/stable/c/78285b53266d6d51fa4ff504a23df03852eba84e
  Patch
• git.kernel.org https://git.kernel.org/stable/c/79a955ea4a2e5ddf4a36328959de0de496419888
  Patch
• git.kernel.org https://git.kernel.org/stable/c/7a9723ec27aff5674f1fd4934608937f1d650980
  Patch
• git.kernel.org https://git.kernel.org/stable/c/a56a6e8589a9b98d8171611fbcc1e45a15fd2455
  Patch
• git.kernel.org https://git.kernel.org/stable/c/b6a079c3b6f95378f26e2aeda520cb3176f7067b
  Patch
• git.kernel.org https://git.kernel.org/stable/c/e40cb34b7f247fe2e366fd192700d1b4f38196ca
  Patch