KVM: Reject device ioctls from processes other than the VM's creator

KVM's API requires thats ioctls must be issued from the same process
that created the VM.  In other words, userspace can play games with a
VM's file descriptors, e.g. fork(), SCM_RIGHTS, etc..., but only the
creator can do anything useful.  Explicitly reject device ioctls that
are issued by a process other than the VM's creator, and update KVM's
API documentation to extend its requirements to device ioctls.

Fixes: 852b6d57 ("kvm: add device control API")
Cc: <stable@vger.kernel.org>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virtual/kvm/api.txt

@@ -13,7 +13,7 @@ of a virtual machine.  The ioctls belong to three classes:
 
  - VM ioctls: These query and set attributes that affect an entire virtual
    machine, for example memory layout.  In addition a VM ioctl is used to
-   create virtual cpus (vcpus).
+   create virtual cpus (vcpus) and devices.
 
    VM ioctls must be issued from the same process (address space) that was
    used to create the VM.
@@ -26,6 +26,11 @@ of a virtual machine.  The ioctls belong to three classes:
    the documentation.  Otherwise, the first ioctl after switching threads
    could see a performance impact.
 
+ - device ioctls: These query and set attributes that control the operation
+   of a single device.
+
+   device ioctls must be issued from the same process (address space) that
+   was used to create the VM.
 
 2. File descriptors
 -------------------
@@ -34,10 +39,11 @@ The kvm API is centered around file descriptors.  An initial
 open("/dev/kvm") obtains a handle to the kvm subsystem; this handle
 can be used to issue system ioctls.  A KVM_CREATE_VM ioctl on this
 handle will create a VM file descriptor which can be used to issue VM
-ioctls.  A KVM_CREATE_VCPU ioctl on a VM fd will create a virtual cpu
-and return a file descriptor pointing to it.  Finally, ioctls on a vcpu
-fd can be used to control the vcpu, including the important task of
-actually running guest code.
+ioctls.  A KVM_CREATE_VCPU or KVM_CREATE_DEVICE ioctl on a VM fd will
+create a virtual cpu or device and return a file descriptor pointing to
+the new resource.  Finally, ioctls on a vcpu or device fd can be used
+to control the vcpu or device.  For vcpus, this includes the important
+task of actually running guest code.
 
 In general file descriptors can be migrated among processes by means
 of fork() and the SCM_RIGHTS facility of unix domain socket.  These

virt/kvm/kvm_main.c

@@ -2905,6 +2905,9 @@ static long kvm_device_ioctl(struct file *filp, unsigned int ioctl,
 {
 	struct kvm_device *dev = filp->private_data;
 
+	if (dev->kvm->mm != current->mm)
+		return -EIO;
+
 	switch (ioctl) {
 	case KVM_SET_DEVICE_ATTR:
 		return kvm_device_ioctl_attr(dev, dev->ops->set_attr, arg);