{"id":"DEBIAN-CVE-2024-26976","details":"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:   \u003cTASK\u003e   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   \u003c/TASK\u003e  ---[ 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 \u003e /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:   \u003cTASK\u003e   __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   \u003c/TASK\u003e  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---","modified":"2026-09-15T09:03:05.996690764Z","published":"2024-05-01T06:15:14.667Z","upstream":["CVE-2024-26976"],"references":[{"type":"ADVISORY","url":"https://security-tracker.debian.org/tracker/CVE-2024-26976"}],"affected":[{"package":{"name":"linux","ecosystem":"Debian:12","purl":"pkg:deb/debian/linux?arch=source&distro=bookworm"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"6.1.85-1"}]}],"versions":["6.1.27-1","6.1.37-1","6.1.38-1","6.1.38-2","6.1.38-2~bpo11+1","6.1.38-3","6.1.38-4","6.1.38-4~bpo11+1","6.1.52-1","6.1.55-1","6.1.55-1~bpo11+1","6.1.64-1","6.1.66-1","6.1.67-1","6.1.69-1","6.1.69-1~bpo11+1","6.1.76-1","6.1.76-1~bpo11+1","6.1.82-1"],"ecosystem_specific":{"urgency":"not yet assigned"},"database_specific":{"source":"https://storage.googleapis.com/osv-test-debian-osv/debian-cve-osv/DEBIAN-CVE-2024-26976.json"}},{"package":{"name":"linux","ecosystem":"Debian:13","purl":"pkg:deb/debian/linux?arch=source&distro=trixie"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"6.7.12-1"}]}],"ecosystem_specific":{"urgency":"not yet assigned"},"database_specific":{"source":"https://storage.googleapis.com/osv-test-debian-osv/debian-cve-osv/DEBIAN-CVE-2024-26976.json"}},{"package":{"name":"linux","ecosystem":"Debian:14","purl":"pkg:deb/debian/linux?arch=source&distro=forky"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"6.7.12-1"}]}],"ecosystem_specific":{"urgency":"not yet assigned"},"database_specific":{"source":"https://storage.googleapis.com/osv-test-debian-osv/debian-cve-osv/DEBIAN-CVE-2024-26976.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H"}]}