Amazon Linux AMI: CVE-2024-26976: Security patch for kernel (ALAS-2024-1942)

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

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