KVM: let kvm_destroy_vm_debugfs clean up vCPU debugfs directories

After commit 63d04348 ("KVM: x86: move kvm_create_vcpu_debugfs after
last failure point") we are creating the pre-vCPU debugfs files
after the creation of the vCPU file descriptor.  This makes it
possible for userspace to reach kvm_vcpu_release before
kvm_create_vcpu_debugfs has finished.  The vcpu->debugfs_dentry
then does not have any associated inode anymore, and this causes
a NULL-pointer dereference in debugfs_create_file.

The solution is simply to avoid removing the files; they are
cleaned up when the VM file descriptor is closed (and that must be
after KVM_CREATE_VCPU returns).  We can stop storing the dentry
in struct kvm_vcpu too, because it is not needed anywhere after
kvm_create_vcpu_debugfs returns.

Reported-by: syzbot+705f4401d5a93a59b87d@syzkaller.appspotmail.com
Fixes: 63d04348 ("KVM: x86: move kvm_create_vcpu_debugfs after last failure point")
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/arm64/kvm/arm.c

@@ -144,11 +144,6 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 	return ret;
 }
 
-int kvm_arch_create_vcpu_debugfs(struct kvm_vcpu *vcpu)
-{
-	return 0;
-}
-
 vm_fault_t kvm_arch_vcpu_fault(struct kvm_vcpu *vcpu, struct vm_fault *vmf)
 {
 	return VM_FAULT_SIGBUS;

arch/x86/kvm/debugfs.c

@@ -43,22 +43,22 @@ static int vcpu_get_tsc_scaling_frac_bits(void *data, u64 *val)
 
 DEFINE_SIMPLE_ATTRIBUTE(vcpu_tsc_scaling_frac_fops, vcpu_get_tsc_scaling_frac_bits, NULL, "%llu\n");
 
-void kvm_arch_create_vcpu_debugfs(struct kvm_vcpu *vcpu)
+void kvm_arch_create_vcpu_debugfs(struct kvm_vcpu *vcpu, struct dentry *debugfs_dentry)
 {
-	debugfs_create_file("tsc-offset", 0444, vcpu->debugfs_dentry, vcpu,
+	debugfs_create_file("tsc-offset", 0444, debugfs_dentry, vcpu,
 			    &vcpu_tsc_offset_fops);
 
 	if (lapic_in_kernel(vcpu))
 		debugfs_create_file("lapic_timer_advance_ns", 0444,
-				    vcpu->debugfs_dentry, vcpu,
+				    debugfs_dentry, vcpu,
 				    &vcpu_timer_advance_ns_fops);
 
 	if (kvm_has_tsc_control) {
 		debugfs_create_file("tsc-scaling-ratio", 0444,
-				    vcpu->debugfs_dentry, vcpu,
+				    debugfs_dentry, vcpu,
 				    &vcpu_tsc_scaling_fops);
 		debugfs_create_file("tsc-scaling-ratio-frac-bits", 0444,
-				    vcpu->debugfs_dentry, vcpu,
+				    debugfs_dentry, vcpu,
 				    &vcpu_tsc_scaling_frac_fops);
 	}
 }

include/linux/kvm_host.h

@@ -318,7 +318,6 @@ struct kvm_vcpu {
 	bool preempted;
 	bool ready;
 	struct kvm_vcpu_arch arch;
-	struct dentry *debugfs_dentry;
 };
 
 static inline int kvm_vcpu_exiting_guest_mode(struct kvm_vcpu *vcpu)
@@ -888,7 +887,7 @@ void kvm_arch_vcpu_postcreate(struct kvm_vcpu *vcpu);
 void kvm_arch_vcpu_destroy(struct kvm_vcpu *vcpu);
 
 #ifdef __KVM_HAVE_ARCH_VCPU_DEBUGFS
-void kvm_arch_create_vcpu_debugfs(struct kvm_vcpu *vcpu);
+void kvm_arch_create_vcpu_debugfs(struct kvm_vcpu *vcpu, struct dentry *debugfs_dentry);
 #endif
 
 int kvm_arch_hardware_enable(void);

virt/kvm/kvm_main.c

@@ -2973,7 +2973,6 @@ static int kvm_vcpu_release(struct inode *inode, struct file *filp)
 {
 	struct kvm_vcpu *vcpu = filp->private_data;
 
-	debugfs_remove_recursive(vcpu->debugfs_dentry);
 	kvm_put_kvm(vcpu->kvm);
 	return 0;
 }
@@ -3000,16 +2999,17 @@ static int create_vcpu_fd(struct kvm_vcpu *vcpu)
 static void kvm_create_vcpu_debugfs(struct kvm_vcpu *vcpu)
 {
 #ifdef __KVM_HAVE_ARCH_VCPU_DEBUGFS
+	struct dentry *debugfs_dentry;
 	char dir_name[ITOA_MAX_LEN * 2];
 
 	if (!debugfs_initialized())
 		return;
 
 	snprintf(dir_name, sizeof(dir_name), "vcpu%d", vcpu->vcpu_id);
-	vcpu->debugfs_dentry = debugfs_create_dir(dir_name,
+	debugfs_dentry = debugfs_create_dir(dir_name,
 					    vcpu->kvm->debugfs_dentry);
 
-	kvm_arch_create_vcpu_debugfs(vcpu);
+	kvm_arch_create_vcpu_debugfs(vcpu, debugfs_dentry);
 #endif
 }