CVE-2024-26976

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() cant return until kvm_put_kvm() finishes, and kvm_put_kvm() cant 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: 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 —[ 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: __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

If kvm_clear_async_pf_completion_queue() actually flushes the workqueue, then theres 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() wont 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 thats 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() cant drop the last reference to the page tables if the vCPU is still alive, i.e. the vCPU wont get stuck tearing down page tables.

Add a helper to do the flushing, specifically to deal with wakeup all work items, as they arent 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—

Weakness

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

Affected Software

Potential Mitigations

• Mitigation of resource exhaustion attacks requires that the target system either:

• The first of these solutions is an issue in itself though, since it may allow attackers to prevent the use of the system by a particular valid user. If the attacker impersonates the valid user, they may be able to prevent the user from accessing the server in question.

• The second solution is simply difficult to effectively institute – and even when properly done, it does not provide a full solution. It simply makes the attack require more resources on the part of the attacker.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/147.html
• https://cwe.mitre.org/data/definitions/227.html
• https://cwe.mitre.org/data/definitions/492.html

References

• https://git.kernel.org/stable/c/3d75b8aa5c29058a512db29da7cbee8052724157
• https://git.kernel.org/stable/c/4f3a3bce428fb439c66a578adc447afce7b4a750
• https://git.kernel.org/stable/c/82e25cc1c2e93c3023da98be282322fc08b61ffb
• https://git.kernel.org/stable/c/83d3c5e309611ef593e2fcb78444fc8ceedf9bac
• https://git.kernel.org/stable/c/a75afe480d4349c524d9c659b1a5a544dbc39a98
• https://git.kernel.org/stable/c/ab2c2f5d9576112ad22cfd3798071cb74693b1f5
• https://git.kernel.org/stable/c/b54478d20375874aeee257744dedfd3e413432ff
• https://git.kernel.org/stable/c/caa9af2e27c275e089d702cfbaaece3b42bca31b
• https://git.kernel.org/stable/c/f8730d6335e5f43d09151fca1f0f41922209a264
• https://git.kernel.org/stable/c/3d75b8aa5c29058a512db29da7cbee8052724157
• https://git.kernel.org/stable/c/4f3a3bce428fb439c66a578adc447afce7b4a750
• https://git.kernel.org/stable/c/82e25cc1c2e93c3023da98be282322fc08b61ffb
• https://git.kernel.org/stable/c/83d3c5e309611ef593e2fcb78444fc8ceedf9bac
• https://git.kernel.org/stable/c/a75afe480d4349c524d9c659b1a5a544dbc39a98
• https://git.kernel.org/stable/c/ab2c2f5d9576112ad22cfd3798071cb74693b1f5
• https://git.kernel.org/stable/c/b54478d20375874aeee257744dedfd3e413432ff
• https://git.kernel.org/stable/c/caa9af2e27c275e089d702cfbaaece3b42bca31b
• https://git.kernel.org/stable/c/f8730d6335e5f43d09151fca1f0f41922209a264
• https://lists.debian.org/debian-lts-announce/2024/06/msg00017.html
• https://lists.debian.org/debian-lts-announce/2024/06/msg00020.html