CVE-2024-26976 Information

May 04, 2024 cve

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: 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 cho 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 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\nwork 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—

Reference

Share on: