CVE-2024-26976

Description

In the Linux kernel, the following vulnerability has been resolved: KVM: Always flush async #PF workqueue when vCPU is being destroyed Always flush the per-vCPU async #PF workqueue when a vCPU is clearing its completion queue, e.g. when a VM and all its vCPUs is being destroyed. KVM must ensure that none of its workqueue callbacks is running when the last reference to the KVM _module_ is put. Gifting a reference to the associated VM prevents the workqueue callback from dereferencing freed vCPU/VM memory, but does not prevent the KVM module from being unloaded before the callback completes. Drop the misguided VM refcount gifting, as calling kvm_put_kvm() from async_pf_execute() if kvm_put_kvm() flushes the async #PF workqueue will result in deadlock. async_pf_execute() can't return until kvm_put_kvm() finishes, and kvm_put_kvm() can't return until async_pf_execute() finishes: WARNING: CPU: 8 PID: 251 at virt/kvm/kvm_main.c:1435 kvm_put_kvm+0x2d/0x320 [kvm] Modules linked in: vhost_net vhost vhost_iotlb tap kvm_intel kvm irqbypass CPU: 8 PID: 251 Comm: kworker/8:1 Tainted: G W 6.6.0-rc1-e7af8d17224a-x86/gmem-vm #119 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015 Workqueue: events async_pf_execute [kvm] RIP: 0010:kvm_put_kvm+0x2d/0x320 [kvm] Call Trace: <TASK> async_pf_execute+0x198/0x260 [kvm] process_one_work+0x145/0x2d0 worker_thread+0x27e/0x3a0 kthread+0xba/0xe0 ret_from_fork+0x2d/0x50 ret_from_fork_asm+0x11/0x20 </TASK> ---[ end trace 0000000000000000 ]--- INFO: task kworker/8:1:251 blocked for more than 120 seconds. Tainted: G W 6.6.0-rc1-e7af8d17224a-x86/gmem-vm #119 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. task:kworker/8:1 state:D stack:0 pid:251 ppid:2 flags:0x00004000 Workqueue: events async_pf_execute [kvm] Call Trace: <TASK> __schedule+0x33f/0xa40 schedule+0x53/0xc0 schedule_timeout+0x12a/0x140 __wait_for_common+0x8d/0x1d0 __flush_work.isra.0+0x19f/0x2c0 kvm_clear_async_pf_completion_queue+0x129/0x190 [kvm] kvm_arch_destroy_vm+0x78/0x1b0 [kvm] kvm_put_kvm+0x1c1/0x320 [kvm] async_pf_execute+0x198/0x260 [kvm] process_one_work+0x145/0x2d0 worker_thread+0x27e/0x3a0 kthread+0xba/0xe0 ret_from_fork+0x2d/0x50 ret_from_fork_asm+0x11/0x20 </TASK> If kvm_clear_async_pf_completion_queue() actually flushes the workqueue, then there's no need to gift async_pf_execute() a reference because all invocations of async_pf_execute() will be forced to complete before the vCPU and its VM are destroyed/freed. And that in turn fixes the module unloading bug as __fput() won't do module_put() on the last vCPU reference until the vCPU has been freed, e.g. if closing the vCPU file also puts the last reference to the KVM module. Note that kvm_check_async_pf_completion() may also take the work item off the completion queue and so also needs to flush the work queue, as the work will not be seen by kvm_clear_async_pf_completion_queue(). Waiting on the workqueue could theoretically delay a vCPU due to waiting for the work to complete, but that's a very, very small chance, and likely a very small delay. kvm_arch_async_page_present_queued() unconditionally makes a new request, i.e. will effectively delay entering the guest, so the remaining work is really just: trace_kvm_async_pf_completed(addr, cr2_or_gpa); __kvm_vcpu_wake_up(vcpu); mmput(mm); and mmput() can't drop the last reference to the page tables if the vCPU is still alive, i.e. the vCPU won't get stuck tearing down page tables. Add a helper to do the flushing, specifically to deal with "wakeup all" work items, as they aren't actually work items, i.e. are never placed in a workqueue. Trying to flush a bogus workqueue entry rightly makes __flush_work() complain (kudos to whoever added that sanity check). Note, commit 5f6de5cbebee ("KVM: Prevent module exit until al ---truncated---

CVSS Metrics

• v3.1•HIGH•Score: 7.8CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

EPSS Trends

Current EPSS score: 0.26%• Percentile: 18%

Techniques & Countermeasures

• CWE-400•Uncontrolled Resource Consumption

  The product does not properly control the allocation and maintenance of a limited resource.

Affected Systems

• debian•linux

  < 5.10.216-1 | < 6.1.85-1 | < 6.7.12-1 | < 6.7.12-1

• ubuntu•linux

  all | < 5.4.0-189.209 | < 5.15.0-116.126 | < 6.8.0-35.35

• ubuntu•linux-allwinner-5.19

  all

• ubuntu•linux-aws

  all | < 5.4.0-1128.138 | < 5.15.0-1065.71 | < 6.8.0-1009.9

• ubuntu•linux-aws-5.0

  all

• ubuntu•linux-aws-5.11

  all

• ubuntu•linux-aws-5.13

  all

• ubuntu•linux-aws-5.15

  < 5.15.0-1065.71~20.04.1

• ubuntu•linux-aws-5.19

  all

• ubuntu•linux-aws-5.3

  all

• ubuntu•linux-aws-5.4

  < 5.4.0-1128.138~18.04.1

• ubuntu•linux-aws-5.8

  all

• ubuntu•linux-aws-6.2

  all

• ubuntu•linux-aws-6.5

  all

• ubuntu•linux-aws-fips

  all | < 5.4.0-1128.138+fips1 | < 5.15.0-1065.71+fips1

• ubuntu•linux-aws-hwe

  all

• ubuntu•linux-azure

  all | all | < 5.4.0-1133.140 | < 5.15.0-1068.77 | < 6.8.0-1008.8

• ubuntu•linux-azure-4.15

  all

• ubuntu•linux-azure-5.11

  all

• ubuntu•linux-azure-5.13

  all

• ubuntu•linux-azure-5.15

  < 5.15.0-1068.77~20.04.1

• ubuntu•linux-azure-5.19

  all

• ubuntu•linux-azure-5.3

  all

• ubuntu•linux-azure-5.4

  < 5.4.0-1133.140~18.04.1

• ubuntu•linux-azure-5.8

  all

• ubuntu•linux-azure-6.2

  all

• ubuntu•linux-azure-6.5

  all

• ubuntu•linux-azure-edge

  all

• ubuntu•linux-azure-fde

  all | < 5.15.0-1068.77.1

• ubuntu•linux-azure-fde-5.19

  all

• ubuntu•linux-azure-fde-6.2

  all

• ubuntu•linux-azure-fips

  all | < 5.4.0-1133.140+fips1 | < 5.15.0-1068.77+fips1

• ubuntu•linux-bluefield

  all | < 5.4.0-1088.95 | < 5.15.0-1047.49

• ubuntu•linux-fips

  all | < 5.4.0-1102.112 | < 5.15.0-115.125+fips1

• ubuntu•linux-gcp

  all | all | < 5.4.0-1132.141 | < 5.15.0-1064.72 | < 6.8.0-1008.9

• ubuntu•linux-gcp-4.15

  all

• ubuntu•linux-gcp-5.11

  all

• ubuntu•linux-gcp-5.13

  all

• ubuntu•linux-gcp-5.15

  < 5.15.0-1065.73~20.04.1

• ubuntu•linux-gcp-5.19

  all

• ubuntu•linux-gcp-5.3

  all

• ubuntu•linux-gcp-5.4

  < 5.4.0-1132.141~18.04.1

• ubuntu•linux-gcp-5.8

  all

• ubuntu•linux-gcp-6.2

  all

• ubuntu•linux-gcp-6.5

  all

• ubuntu•linux-gcp-fips

  all | < 5.4.0-1132.141+fips1 | < 5.15.0-1064.72+fips1

• ubuntu•linux-gke

  all | < 5.15.0-1062.68 | < 6.8.0-1004.7

• ubuntu•linux-gke-4.15

  all

• ubuntu•linux-gke-5.15

  all

• ubuntu•linux-gke-5.4

  all

Showing first 50 affected entries in server-rendered view.

References (33)

• https://git.kernel.org/stable/c/ab2c2f5d9576112ad22cfd3798071cb74693b1f5
• https://git.kernel.org/stable/c/82e25cc1c2e93c3023da98be282322fc08b61ffb
• https://git.kernel.org/stable/c/f8730d6335e5f43d09151fca1f0f41922209a264
• https://git.kernel.org/stable/c/83d3c5e309611ef593e2fcb78444fc8ceedf9bac
• https://git.kernel.org/stable/c/b54478d20375874aeee257744dedfd3e413432ff
• https://git.kernel.org/stable/c/a75afe480d4349c524d9c659b1a5a544dbc39a98
• https://git.kernel.org/stable/c/4f3a3bce428fb439c66a578adc447afce7b4a750
• https://git.kernel.org/stable/c/caa9af2e27c275e089d702cfbaaece3b42bca31b
• https://git.kernel.org/stable/c/3d75b8aa5c29058a512db29da7cbee8052724157
• https://lists.debian.org/debian-lts-announce/2024/06/msg00017.html
• https://lists.debian.org/debian-lts-announce/2024/06/msg00020.html
• https://ubuntu.com/security/CVE-2024-26976
• https://www.cve.org/CVERecord?id=CVE-2024-26976
• https://git.kernel.org/linus/3d75b8aa5c29058a512db29da7cbee8052724157
• https://ubuntu.com/security/notices/USN-6816-1
• https://ubuntu.com/security/notices/USN-6817-1
• https://ubuntu.com/security/notices/USN-6817-2
• https://ubuntu.com/security/notices/USN-6817-3
• https://ubuntu.com/security/notices/USN-6878-1
• https://ubuntu.com/security/notices/USN-6896-1
• https://ubuntu.com/security/notices/USN-6898-1
• https://ubuntu.com/security/notices/USN-6896-2
• https://ubuntu.com/security/notices/USN-6898-2
• https://ubuntu.com/security/notices/USN-6896-3
• https://ubuntu.com/security/notices/USN-6898-3
• https://ubuntu.com/security/notices/USN-6896-4
• https://ubuntu.com/security/notices/USN-6896-5
• https://ubuntu.com/security/notices/USN-6898-4
• https://ubuntu.com/security/notices/USN-6917-1
• https://ubuntu.com/security/notices/USN-6919-1
• https://ubuntu.com/security/notices/USN-6927-1
• https://ubuntu.com/security/notices/USN-7019-1
• https://security-tracker.debian.org/tracker/CVE-2024-26976