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—