vmm: Add privilege checks to vmmctl operations

In preparation for supporting creation of VMs by unprivileged users, add
some restrictions:
- Disallow creation of non-transient VMs by unprivileged users.  That
  is, if an unprivileged user creates a VM, the VM must be destroyed
  automatically once the last fd referencing it is gone.
- Disallow destroying VMs created by a different user, unless the caller
  has the PRIV_VMM_DESTROY privilege.

Reviewed by:	bnovkov
MFC after:	2 months
Sponsored by:	The FreeBSD Foundation
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D54740

sys/dev/vmm/vmm_dev.c

@@ -898,6 +898,7 @@ vmmdev_lookup_and_destroy(const char *name, struct ucred *cred)
 {
 	struct cdev *cdev;
 	struct vmmdev_softc *sc;
+	int error;
 
 	sx_xlock(&vmmdev_mtx);
 	sc = vmmdev_lookup(name, cred);
@@ -906,6 +907,16 @@ vmmdev_lookup_and_destroy(const char *name, struct ucred *cred)
 		return (EINVAL);
 	}
 
+	/*
+	 * Only the creator of a VM or a privileged user can destroy it.
+	 */
+	if ((cred->cr_uid != sc->ucred->cr_uid ||
+	     cred->cr_prison != sc->ucred->cr_prison) &&
+	    (error = priv_check_cred(cred, PRIV_VMM_DESTROY)) != 0) {
+		sx_xunlock(&vmmdev_mtx);
+		return (error);
+	}
+
 	/*
 	 * Setting 'sc->cdev' to NULL is used to indicate that the VM
 	 * is scheduled for destruction.
@@ -992,6 +1003,16 @@ vmmdev_create(const char *name, uint32_t flags, struct ucred *cred)
 		return (EEXIST);
 	}
 
+	/*
+	 * Unprivileged users can only create VMs that will be automatically
+	 * destroyed when the creating descriptor is closed.
+	 */
+	if ((flags & VMMCTL_CREATE_DESTROY_ON_CLOSE) == 0 &&
+	    (error = priv_check_cred(cred, PRIV_VMM_CREATE)) != 0) {
+		sx_xunlock(&vmmdev_mtx);
+		return (error);
+	}
+
 	if (!chgvmmcnt(cred->cr_ruidinfo, 1, vm_maxvmms)) {
 		sx_xunlock(&vmmdev_mtx);
 		return (ENOMEM);

sys/sys/priv.h

@@ -539,11 +539,13 @@
  * vmm privileges.
  */
 #define	PRIV_VMM_PPTDEV		710	/* Can manipulate ppt devices. */
+#define	PRIV_VMM_CREATE		711	/* Can create non-temporal VMs. */
+#define	PRIV_VMM_DESTROY	712	/* Can destroy other users' VMs. */
 
 /*
  * Track end of privilege list.
  */
-#define	_PRIV_HIGHEST		711
+#define	_PRIV_HIGHEST		713
 
 /*
  * Validate that a named privilege is known by the privilege system.  Invalid