| Message ID | 20220816053937.2477106-2-seanjc@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: kvm_create_vm() bug fixes and cleanup | expand |
On Tue, Aug 16, 2022 at 05:39:35AM +0000, Sean Christopherson wrote: > Properly unwind VM creation if kvm_create_vm_debugfs() fails. A recent > change to invoke kvm_create_vm_debug() in kvm_create_vm() was led astray typo: kvm_create_vm_debugfs() > by buggy try_get_module() handling adding by commit 5f6de5cbebee ("KVM: > Prevent module exit until all VMs are freed"). The debugfs error path > effectively inherits the bad error path of try_module_get(), e.g. KVM > leaves the to-be-free VM on vm_list even though KVM appears to do the > right thing by calling module_put() and falling through. > > Opportunistically hoist kvm_create_vm_debugfs() above the call to > kvm_arch_post_init_vm() so that the "post-init" arch hook is actually > invoked after the VM is initialized (ignoring kvm_coalesced_mmio_init() > for the moment). x86 is the only non-nop implementation of the post-init > hook, and it doesn't allocate/initialize any objects that are reachable > via debugfs code (spawns a kthread worker for the NX huge page mitigation). > > Leave the buggy try_get_module() alone for now, it will be fixed in a > separate commit. > > Fixes: b74ed7a68ec1 ("KVM: Actually create debugfs in kvm_create_vm()") > Reported-by: syzbot+744e173caec2e1627ee0@syzkaller.appspotmail.com > Cc: Oliver Upton <oliver.upton@linux.dev> > Signed-off-by: Sean Christopherson <seanjc@google.com> Fun times! Thanks for the fix Sean. Reviewed-by: Oliver Upton <oliver.upton@linux.dev> -- Best, Oliver
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 515dfe9d3bcf..ee5f48cc100b 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -1211,9 +1211,13 @@ static struct kvm *kvm_create_vm(unsigned long type, const char *fdname) if (r) goto out_err_no_mmu_notifier; + r = kvm_create_vm_debugfs(kvm, fdname); + if (r) + goto out_err_no_debugfs; + r = kvm_arch_post_init_vm(kvm); if (r) - goto out_err_mmu_notifier; + goto out_err; mutex_lock(&kvm_lock); list_add(&kvm->vm_list, &vm_list); @@ -1229,18 +1233,14 @@ static struct kvm *kvm_create_vm(unsigned long type, const char *fdname) */ if (!try_module_get(kvm_chardev_ops.owner)) { r = -ENODEV; - goto out_err_mmu_notifier; - } - - r = kvm_create_vm_debugfs(kvm, fdname); - if (r) goto out_err; + } return kvm; out_err: - module_put(kvm_chardev_ops.owner); -out_err_mmu_notifier: + kvm_destroy_vm_debugfs(kvm); +out_err_no_debugfs: #if defined(CONFIG_MMU_NOTIFIER) && defined(KVM_ARCH_WANT_MMU_NOTIFIER) if (kvm->mmu_notifier.ops) mmu_notifier_unregister(&kvm->mmu_notifier, current->mm);
Properly unwind VM creation if kvm_create_vm_debugfs() fails. A recent change to invoke kvm_create_vm_debug() in kvm_create_vm() was led astray by buggy try_get_module() handling adding by commit 5f6de5cbebee ("KVM: Prevent module exit until all VMs are freed"). The debugfs error path effectively inherits the bad error path of try_module_get(), e.g. KVM leaves the to-be-free VM on vm_list even though KVM appears to do the right thing by calling module_put() and falling through. Opportunistically hoist kvm_create_vm_debugfs() above the call to kvm_arch_post_init_vm() so that the "post-init" arch hook is actually invoked after the VM is initialized (ignoring kvm_coalesced_mmio_init() for the moment). x86 is the only non-nop implementation of the post-init hook, and it doesn't allocate/initialize any objects that are reachable via debugfs code (spawns a kthread worker for the NX huge page mitigation). Leave the buggy try_get_module() alone for now, it will be fixed in a separate commit. Fixes: b74ed7a68ec1 ("KVM: Actually create debugfs in kvm_create_vm()") Reported-by: syzbot+744e173caec2e1627ee0@syzkaller.appspotmail.com Cc: Oliver Upton <oliver.upton@linux.dev> Signed-off-by: Sean Christopherson <seanjc@google.com> --- virt/kvm/kvm_main.c | 16 ++++++++-------- 1 file changed, 8 insertions(+), 8 deletions(-)