The patch below does not apply to the 5.4-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
From 405294f29faee5de8c10cb9d4a90e229c2835279 Mon Sep 17 00:00:00 2001 From: Sean Christopherson seanjc@google.com Date: Tue, 16 Aug 2022 05:39:36 +0000 Subject: [PATCH] KVM: Unconditionally get a ref to /dev/kvm module when creating a VM
Unconditionally get a reference to the /dev/kvm module when creating a VM instead of using try_get_module(), which will fail if the module is in the process of being forcefully unloaded. The error handling when try_get_module() fails doesn't properly unwind all that has been done, e.g. doesn't call kvm_arch_pre_destroy_vm() and doesn't remove the VM from the global list. Not removing VMs from the global list tends to be fatal, e.g. leads to use-after-free explosions.
The obvious alternative would be to add proper unwinding, but the justification for using try_get_module(), "rmmod --wait", is completely bogus as support for "rmmod --wait", i.e. delete_module() without O_NONBLOCK, was removed by commit 3f2b9c9cdf38 ("module: remove rmmod --wait option.") nearly a decade ago.
It's still possible for try_get_module() to fail due to the module dying (more like being killed), as the module will be tagged MODULE_STATE_GOING by "rmmod --force", i.e. delete_module(..., O_TRUNC), but playing nice with forced unloading is an exercise in futility and gives a falsea sense of security. Using try_get_module() only prevents acquiring _new_ references, it doesn't magically put the references held by other VMs, and forced unloading doesn't wait, i.e. "rmmod --force" on KVM is all but guaranteed to cause spectacular fireworks; the window where KVM will fail try_get_module() is tiny compared to the window where KVM is building and running the VM with an elevated module refcount.
Addressing KVM's inability to play nice with "rmmod --force" is firmly out-of-scope. Forcefully unloading any module taints kernel (for obvious reasons) _and_ requires the kernel to be built with CONFIG_MODULE_FORCE_UNLOAD=y, which is off by default and comes with the amusing disclaimer that it's "mainly for kernel developers and desperate users". In other words, KVM is free to scoff at bug reports due to using "rmmod --force" while VMs may be running.
Fixes: 5f6de5cbebee ("KVM: Prevent module exit until all VMs are freed") Cc: stable@vger.kernel.org Cc: David Matlack dmatlack@google.com Signed-off-by: Sean Christopherson seanjc@google.com Message-Id: 20220816053937.2477106-3-seanjc@google.com Signed-off-by: Paolo Bonzini pbonzini@redhat.com
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index ee5f48cc100b..15e304e059d4 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -1134,6 +1134,9 @@ static struct kvm *kvm_create_vm(unsigned long type, const char *fdname) if (!kvm) return ERR_PTR(-ENOMEM);
+ /* KVM is pinned via open("/dev/kvm"), the fd passed to this ioctl(). */ + __module_get(kvm_chardev_ops.owner); + KVM_MMU_LOCK_INIT(kvm); mmgrab(current->mm); kvm->mm = current->mm; @@ -1226,16 +1229,6 @@ static struct kvm *kvm_create_vm(unsigned long type, const char *fdname) preempt_notifier_inc(); kvm_init_pm_notifier(kvm);
- /* - * When the fd passed to this ioctl() is opened it pins the module, - * but try_module_get() also prevents getting a reference if the module - * is in MODULE_STATE_GOING (e.g. if someone ran "rmmod --wait"). - */ - if (!try_module_get(kvm_chardev_ops.owner)) { - r = -ENODEV; - goto out_err; - } - return kvm;
out_err: @@ -1259,6 +1252,7 @@ static struct kvm *kvm_create_vm(unsigned long type, const char *fdname) out_err_no_srcu: kvm_arch_free_vm(kvm); mmdrop(current->mm); + module_put(kvm_chardev_ops.owner); return ERR_PTR(r); }