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() 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---