path: root/sys/kern/kern_unveil.c

/*	$OpenBSD: kern_unveil.c,v 1.13 2018/08/11 16:16:07 beck Exp $	*/

/*
 * Copyright (c) 2017-2018 Bob Beck <beck@openbsd.org>
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

#include <sys/param.h>

#include <sys/mount.h>
#include <sys/proc.h>
#include <sys/namei.h>
#include <sys/pool.h>
#include <sys/vnode.h>
#include <sys/ktrace.h>
#include <sys/types.h>
#include <sys/malloc.h>
#include <sys/tree.h>

#include <sys/conf.h>
#include <sys/syscall.h>
#include <sys/syscallargs.h>
#include <sys/systm.h>

#include <sys/pledge.h>

/* #define DEBUG_UNVEIL */

#define UNVEIL_MAX_VNODES	128
#define UNVEIL_MAX_NAMES	128

static inline int
unvname_compare(const struct unvname *n1, const struct unvname *n2)
{
	if (n1->un_namesize == n2->un_namesize)
		return (memcmp(n1->un_name, n2->un_name, n1->un_namesize));
	else
		return (n1->un_namesize - n2->un_namesize);
}

struct unvname *
unvname_new(const char *name, size_t size, u_char flags)
{
	struct unvname *ret = malloc(sizeof(struct unvname), M_PROC, M_WAITOK);
	ret->un_name = malloc(size, M_PROC, M_WAITOK);
	memcpy(ret->un_name, name, size);
	ret->un_namesize = size;
	ret->un_flags = flags;
	return ret;
}

void
unveil_free_traversed_vnodes(struct nameidata *ndp)
{
	if (ndp->ni_tvpsize) {
		size_t i;

		for (i = 0; i < ndp->ni_tvpend; i++)
			vrele(ndp->ni_tvp[i]); /* ref for being in list */
		free(ndp->ni_tvp, M_PROC, ndp->ni_tvpsize * sizeof(struct vnode *));
		ndp->ni_tvpsize = 0;
		ndp->ni_tvpend = 0;
	}
}

void
unveil_save_traversed_vnode(struct nameidata *ndp, struct vnode *vp)
{
	if (ndp->ni_tvpsize == 0) {
		ndp->ni_tvp = mallocarray(MAXPATHLEN, sizeof(struct vnode *),
		    M_PROC, M_WAITOK);
		ndp->ni_tvpsize = MAXPATHLEN;
	}
	/* This should be limited by MAXPATHLEN on a single lookup */
	KASSERT(ndp->ni_tvpsize > ndp->ni_tvpend);
	vref(vp); /* ref for being in the list */
	ndp->ni_tvp[ndp->ni_tvpend++] = vp;
}

void
unvname_delete(struct unvname *name)
{
	free(name->un_name, M_PROC, name->un_namesize);;
	free(name, M_PROC, sizeof(struct unvname));
}

RBT_PROTOTYPE(unvname_rbt, unvname, un_rbt, unvname_compare);
RBT_GENERATE(unvname_rbt, unvname, un_rbt, unvname_compare);

int
unveil_delete_names(struct unveil *uv)
{
	struct unvname *unvn, *next;
	int ret = 0;

	rw_enter_write(&uv->uv_lock);
	RBT_FOREACH_SAFE(unvn, unvname_rbt, &uv->uv_names, next) {
		RBT_REMOVE(unvname_rbt, &uv->uv_names, unvn);
		unvname_delete(unvn);
		ret++;
	}
	rw_exit_write(&uv->uv_lock);
#ifdef DEBUG_UNVEIL
	printf("deleted %d names\n", ret);
#endif
	return ret;
}

void
unveil_add_name(struct unveil *uv, char *name, u_char flags)
{
	struct unvname *unvn;

	rw_enter_write(&uv->uv_lock);
	unvn = unvname_new(name, strlen(name) + 1, flags);
	RBT_INSERT(unvname_rbt, &uv->uv_names, unvn);
	rw_exit_write(&uv->uv_lock);
#ifdef DEBUG_UNVEIL
	printf("added name %s underneath vnode %p\n", name, uv->uv_vp);
#endif
}

struct unvname *
unveil_namelookup(struct unveil *uv, char *name)
{
	struct unvname n, *ret = NULL;

	rw_enter_read(&uv->uv_lock);

#ifdef DEBUG_UNVEIL
	printf("unveil_namelookup: looking up name %s (%p) in vnode %p\n",
	    name, name, uv->uv_vp);
#endif

	KASSERT(uv->uv_vp != NULL);

	n.un_name = name;
	n.un_namesize = strlen(name) + 1;

	ret = RBT_FIND(unvname_rbt, &uv->uv_names, &n);

	rw_exit_read(&uv->uv_lock);

#ifdef DEBUG_UNVEIL
	if (ret == NULL)
		printf("unveil_namelookup: no match for name %s in vnode %p\n",
		    name, uv->uv_vp);
	else
		printf("unveil_namelookup: matched name %s in vnode %p\n",
		    name, uv->uv_vp);
#endif
	return ret;
}

void
unveil_destroy(struct process *ps)
{
	size_t i;

	for (i = 0; ps->ps_uvpaths != NULL && i < ps->ps_uvvcount; i++) {
		struct unveil *uv = ps->ps_uvpaths + i;

		struct vnode *vp = uv->uv_vp;
		/* skip any vnodes zapped by unveil_removevnode */
		if (vp != NULL) {
			vp->v_uvcount--;
#ifdef DEBUG_UNVEIL
			printf("unveil: %s(%d): removing vnode %p uvcount %d "
			    "in position %ld\n",
			    ps->ps_comm, ps->ps_pid, vp, vp->v_uvcount, i);
#endif
			vrele(vp);
		}
		ps->ps_uvncount -= unveil_delete_names(uv);
		uv->uv_vp = NULL;
		uv->uv_flags = 0;
	}

	KASSERT(ps->ps_uvncount == 0);
	free(ps->ps_uvpaths, M_PROC, UNVEIL_MAX_VNODES *
	    sizeof(struct unveil));
	ps->ps_uvvcount = 0;
	ps->ps_uvpaths = NULL;
}

void
unveil_copy(struct process *parent, struct process *child)
{
	size_t i;

	if (parent->ps_uvvcount == 0)
		return;

	child->ps_uvpaths = mallocarray(UNVEIL_MAX_VNODES, sizeof(struct unveil),
	    M_PROC, M_WAITOK|M_ZERO);

	child->ps_uvncount = 0;
	for (i = 0; parent->ps_uvpaths != NULL && i < parent->ps_uvvcount;
	     i++) {
		struct unveil *from = parent->ps_uvpaths + i;
		struct unveil *to = child->ps_uvpaths + i;
		struct unvname *unvn, *next;

		to->uv_vp = from->uv_vp;
		if (to->uv_vp != NULL) {
			vref(to->uv_vp);
			to->uv_vp->v_uvcount++;
		}
		rw_init(&to->uv_lock, "unveil");
		RBT_INIT(unvname_rbt, &to->uv_names);
		rw_enter_read(&from->uv_lock);
		RBT_FOREACH_SAFE(unvn, unvname_rbt, &from->uv_names, next) {
			unveil_add_name(&child->ps_uvpaths[i], unvn->un_name,
			    unvn->un_flags);
			child->ps_uvncount++;
		}
		rw_exit_read(&from->uv_lock);
		to->uv_flags = from->uv_flags;
	}
	child->ps_uvvcount = parent->ps_uvvcount;
	if (parent->ps_uvpcwd)
		child->ps_uvpcwd = child->ps_uvpaths +
		    (parent->ps_uvpcwd - parent->ps_uvpaths);
	child->ps_uvpcwdgone = parent->ps_uvpcwdgone;
	child->ps_uvdone = parent->ps_uvdone;
	child->ps_uvshrink = parent->ps_uvshrink;
}

struct unveil *
unveil_lookup(struct vnode *vp, struct proc *p)
{
	struct process *pr = p->p_p;
	struct unveil *uv = pr->ps_uvpaths;
	ssize_t l, r;

	if (vp->v_uvcount == 0)
		return NULL;

	/*
	 * shrink if told to do so to remove dead vnodes.
	 */
	if (pr->ps_uvshrink) {
		size_t i = 0, j;

		while (i < pr->ps_uvvcount) {
			if (uv[i].uv_vp == NULL)  {
				pr->ps_uvncount -= unveil_delete_names(&uv[i]);
				for (j = i + 1; j < pr->ps_uvvcount; j++)
					uv[j - 1] = uv[j];
				pr->ps_uvvcount--;
			}
			i++;
		}
		pr->ps_uvshrink = 0;
	}

	if (pr->ps_uvvcount == 0)
		return NULL;

	/* clear the cwd unveil when we .. past it */
	if (pr->ps_uvpcwd && (vp == pr->ps_uvpcwd->uv_vp)) {
#ifdef DEBUG_UNVEIL
		printf("unveil: %s(%d): nuking cwd traversing vnode %p\n",
		    p->p_p->ps_comm, p->p_p->ps_pid, vp);
#endif
		p->p_p->ps_uvpcwd = NULL;
		p->p_p->ps_uvpcwdgone = 0;
	}
#ifdef DEBUG_UNVEIL
	else {
		if (pr->ps_uvpcwd) {
			printf("unveil: %s(%d): did not nuke cwd because %p != %p\n",
			    p->p_p->ps_comm, p->p_p->ps_pid, vp, pr->ps_uvpcwd->uv_vp);
		} else
			printf("unveil: %s(%d): cwd is null\n",
 			    p->p_p->ps_comm, p->p_p->ps_pid);
	}
#endif

	l = 0;
	r = pr->ps_uvvcount - 1;
	while (l <= r) {
		size_t m = l + (r - l)/2;
#ifdef DEBUG_UNVEIL
		printf("unveil: checking vnode %p vs. unveil vnode %p\n",
		   vp, uv[m].uv_vp);
#endif
		if (vp == uv[m].uv_vp) {
			KASSERT(uv[m].uv_vp->v_uvcount > 0);
			KASSERT(uv[m].uv_vp->v_usecount > 0);
			return &uv[m];
		}
		if (vp > uv[m].uv_vp)
			l = m + 1;
		else
			r = m - 1;
	}
	return NULL;
}

int
unveil_parsepermissions(const char *permissions, u_char *perms)
{
	size_t i = 0;
	char c;

	*perms = 0;
	while ((c = permissions[i++]) != '\0') {
		switch (c) {
		case 'r':
			*perms |= UNVEIL_READ;
			break;
		case 'w':
			*perms |= UNVEIL_WRITE;
			break;
		case 'x':
			*perms |= UNVEIL_EXEC;
			break;
		case 'c':
			*perms |= UNVEIL_CREATE;
			break;
		default:
			return -1;
		}
	}
	return 0;
}

int
unveil_setflags(u_char *flags, u_char nflags)
{
#if 0
	if (((~(*flags)) & nflags) != 0) {
#ifdef DEBUG_UNVEIL
		printf("Flags escalation %llX -> %llX\n", *flags, nflags);
#endif
		return 1;
	}
#endif
	*flags = nflags;
	return 1;
}

struct unveil *
unveil_add_vnode(struct process *pr, struct vnode *vp)
{
	struct unveil *uv = NULL;
	ssize_t i;

	KASSERT(pr->ps_uvvcount < UNVEIL_MAX_VNODES);

	for (i = pr->ps_uvvcount;
	     i > 0 && pr->ps_uvpaths[i - 1].uv_vp > vp;
	     i--)
		pr->ps_uvpaths[i] = pr->ps_uvpaths[i - 1];

	uv = &pr->ps_uvpaths[i];
	rw_init(&uv->uv_lock, "unveil");
	RBT_INIT(unvname_rbt, &uv->uv_names);
	uv->uv_vp = vp;
	uv->uv_flags = 0;
	pr->ps_uvvcount++;
	return (uv);
}

void
unveil_add_traversed_vnodes(struct proc *p, struct nameidata *ndp)
{
	/*
	 * Add the traversed vnodes with the UNVEIL_INSPECT flag
	 * if they are not already present to allow traversal
	 * operations such as access and stat. This lets
	 * TOCTOU fans that call access on all components of
	 * an unveil'ed path before the final operation
	 * work.
	 */
	struct unveil *uv;

	if (ndp->ni_tvpsize) {
		size_t i;

		for (i = 0; i < ndp->ni_tvpend; i++) {
			struct vnode *vp = ndp->ni_tvp[i];
			if (unveil_lookup(vp, p) == NULL) {
				vref(vp);
				vp->v_uvcount++;
				uv = unveil_add_vnode(p->p_p, vp);
				uv->uv_flags = UNVEIL_INSPECT;
			}
		}
	}
}

int
unveil_add(struct proc *p, struct nameidata *ndp, const char *permissions)
{
	struct process *pr = p->p_p;
	struct vnode *vp;
	struct unveil *uv;
	int directory_add;
	int ret = EINVAL;
	u_char flags;

	KASSERT(ISSET(ndp->ni_cnd.cn_flags, HASBUF)); /* must have SAVENAME */

	if (unveil_parsepermissions(permissions, &flags) == -1)
		goto done;

	if (pr->ps_uvpaths == NULL) {
		pr->ps_uvpaths = mallocarray(UNVEIL_MAX_VNODES,
		    sizeof(struct unveil), M_PROC, M_WAITOK|M_ZERO);
	}

	if ((pr->ps_uvvcount + ndp->ni_tvpend) >= UNVEIL_MAX_VNODES ||
	    pr->ps_uvncount >= UNVEIL_MAX_NAMES) {
		ret = E2BIG;
		goto done;
	}

	/* Are we a directory? or something else */
	directory_add = ndp->ni_vp != NULL && ndp->ni_vp->v_type == VDIR;

	if (directory_add)
		vp = ndp->ni_vp;
	else
		vp = ndp->ni_dvp;

	KASSERT(vp->v_type == VDIR);
	vref(vp);
	vp->v_uvcount++;
	if ((uv = unveil_lookup(vp, p)) != NULL) {
		/*
		 * We already have unveiled this directory
		 * vnode
		 */
		vp->v_uvcount--;
		vrele(vp);

		/*
		 * If we are adding a directory which was already
		 * unveiled containing only specific terminals,
		 * unrestrict it.
		 */
		if (directory_add) {
#ifdef DEBUG_UNVEIL
			printf("unveil: %s(%d): updating directory vnode %p"
			    " to unrestricted uvcount %d\n",
			    pr->ps_comm, pr->ps_pid, vp, vp->v_uvcount);
#endif
			if (!unveil_setflags(&uv->uv_flags, flags))
				ret = EPERM;
			else
				ret = 0;
			goto done;
		}

		/*
		 * If we are adding a terminal that is already unveiled, just
		 * replace the flags and we are done
		 */
		if (!directory_add) {
			struct unvname *tname;
			if ((tname = unveil_namelookup(uv,
			    ndp->ni_cnd.cn_nameptr)) != NULL) {
#ifdef DEBUG_UNVEIL
				printf("unveil: %s(%d): changing flags for %s"
				    "in vnode %p, uvcount %d\n",
				    pr->ps_comm, pr->ps_pid, tname->un_name, vp,
				    vp->v_uvcount);
#endif
				if (!unveil_setflags(&tname->un_flags, flags))
					ret = EPERM;
				else
					ret = 0;
				goto done;
			}
		}

	} else {
		/*
		 * New unveil involving this directory vnode.
		 */
		uv = unveil_add_vnode(pr, vp);
	}

	/*
	 * At this stage with have a unveil in uv with a vnode for a
	 * directory. If the component we are adding is a directory,
	 * we are done. Otherwise, we add the component name the name
	 * list in uv.
	 */

	if (directory_add) {
		uv->uv_flags = flags;
		ret = 0;
#ifdef DEBUG_UNVEIL
		printf("unveil: %s(%d): added unrestricted directory vnode %p"
		    ", uvcount %d\n",
		    pr->ps_comm, pr->ps_pid, vp, vp->v_uvcount);
#endif
		goto done;
	}

	unveil_add_name(uv, ndp->ni_cnd.cn_nameptr, flags);
	pr->ps_uvncount++;
	ret = 0;

#ifdef DEBUG_UNVEIL
	printf("unveil: %s(%d): added name %s beneath %s vnode %p,"
	    " uvcount %d\n",
	    pr->ps_comm, pr->ps_pid, ndp->ni_cnd.cn_nameptr,
	    uv->uv_flags ? "unrestricted" : "restricted",
	    vp, vp->v_uvcount);
#endif

 done:
	if (ret == 0)
		unveil_add_traversed_vnodes(p, ndp);
	unveil_free_traversed_vnodes(ndp);
	pool_put(&namei_pool, ndp->ni_cnd.cn_pnbuf);
	return ret;
}

/*
 * XXX this will probably change.
 * XXX collapse down later once debug surely unneded
 */
int
unveil_flagmatch(struct nameidata *ni, u_char flags)
{
	if (flags == 0) {
#ifdef DEBUG_UNVEIL
		printf("All operations forbidden for 0 flags\n");
#endif
		return 0;
	}
	if (ni->ni_unveil & UNVEIL_READ) {
		if ((flags & UNVEIL_READ) == 0) {
#ifdef DEBUG_UNVEIL
			printf("unveil lacks UNVEIL_READ\n");
#endif
			return 0;
		}
	}
	if (ni->ni_unveil & UNVEIL_WRITE) {
		if ((flags & UNVEIL_WRITE) == 0) {
#ifdef DEBUG_UNVEIL
			printf("unveil lacks UNVEIL_WRITE\n");
#endif
			return 0;
		}
	}
	if (ni->ni_unveil & UNVEIL_EXEC) {
		if ((flags & UNVEIL_EXEC) == 0) {
#ifdef DEBUG_UNVEIL
			printf("unveil lacks UNVEIL_EXEC\n");
#endif
			return 0;
		}
	}
	if (ni->ni_unveil & UNVEIL_CREATE) {
		if ((flags & UNVEIL_CREATE) == 0) {
#ifdef DEBUG_UNVEIL
			printf("unveil lacks UNVEIL_CREATE\n");
#endif
			return 0;
		}
	}
	if (ni->ni_unveil & UNVEIL_INSPECT) {
#ifdef DEBUG_UNVEIL
		printf("any unveil allows UNVEIL_INSPECT\n");
#endif
	}
	return 1;
}

/*
 * unveil checking - for component directories in a namei lookup.
 */
void
unveil_check_component(struct proc *p, struct nameidata *ni, struct vnode *dp)
{
	struct unveil *uv = NULL;

	if (ni->ni_pledge != PLEDGE_UNVEIL) {
		if ((ni->ni_cnd.cn_flags & BYPASSUNVEIL) == 0 &&
		    ! (ni->ni_cnd.cn_flags & ISDOTDOT) &&
		    (uv = unveil_lookup(dp, p)) != NULL) {
			/* if directory flags match, it's a match */
			if (unveil_flagmatch(ni, uv->uv_flags)) {
				if (uv->uv_flags & UNVEIL_USERSET) {
					ni->ni_unveil_match = uv;
#ifdef DEBUG_UNVEIL
					printf("unveil: %s(%d): component directory match"
					    " for vnode %p\n",
					    p->p_p->ps_comm, p->p_p->ps_pid, dp);

#endif
				}
			}
		}
	} else
		unveil_save_traversed_vnode(ni, dp);
}

/*
 * unveil checking - only done after namei lookup has succeeded on
 * the last compoent of a namei lookup.
 */
int
unveil_check_final(struct proc *p, struct nameidata *ni)
{
	struct unveil *uv;
	struct unvname *tname = NULL;

	if (ni->ni_pledge == PLEDGE_UNVEIL ||
	    p->p_p->ps_uvpaths == NULL)
		return (0);

	if (ni->ni_cnd.cn_flags & BYPASSUNVEIL) {
#ifdef DEBUG_UNVEIL
		printf("unveil: %s(%d): BYPASSUNVEIL.\n",
		    p->p_p->ps_comm, p->p_p->ps_pid);
#endif
		CLR(ni->ni_pledge, PLEDGE_STATLIE);
		return (0);
	}
	if (ni->ni_vp != NULL && ni->ni_vp->v_type == VDIR) {
		uv = unveil_lookup(ni->ni_vp, p);
		if (uv == NULL) {
#ifdef DEBUG_UNVEIL
			printf("unveil: %s(%d) no match for vnode %p\n",
			    p->p_p->ps_comm, p->p_p->ps_pid, ni->ni_vp);
#endif
			goto done;
		}
		if (!unveil_flagmatch(ni, uv->uv_flags)) {
#ifdef DEBUG_UNVEIL
			printf("unveil: %s(%d) flag mismatch for directory"
			    " vnode %p\n",
			    p->p_p->ps_comm, p->p_p->ps_pid, ni->ni_vp);
#endif
			return EACCES;
		}
	} else {
		uv = unveil_lookup(ni->ni_dvp, p);
		if (uv == NULL) {
#ifdef DEBUG_UNVEIL
			printf("unveil: %s(%d) no match for directory"
			    " vnode %p\n",
			    p->p_p->ps_comm, p->p_p->ps_pid, ni->ni_dvp);
#endif
			goto done;
		}
		if ((tname = unveil_namelookup(uv, ni->ni_cnd.cn_nameptr))
		    == NULL) {
#ifdef DEBUG_UNVEIL
			printf("unveil: %s(%d) no match for terminal '%s' in "
			    "directory vnode %p\n",
			    p->p_p->ps_comm, p->p_p->ps_pid,
			    ni->ni_cnd.cn_nameptr, ni->ni_dvp);
#endif
			uv = NULL;
			goto done;
		}
		if (!unveil_flagmatch(ni, tname->un_flags)) {
#ifdef DEBUG_UNVEIL
			printf("unveil: %s(%d) flag mismatch for terminal '%s'\n",
			    p->p_p->ps_comm, p->p_p->ps_pid, tname->un_name);
#endif
			return EACCES;
		}
	}
	ni->ni_unveil_match = uv;
done:
	if (ni->ni_unveil_match) {
#ifdef DEBUG_UNVEIL
		printf("unveil: %s(%d): matched \"%s\" underneath/at vnode %p\n",
		    p->p_p->ps_comm, p->p_p->ps_pid, ni->ni_cnd.cn_nameptr,
		    ni->ni_unveil_match->uv_vp);
#endif
		return (0);
	} else if (p->p_p->ps_uvpcwd) {
		ni->ni_unveil_match = p->p_p->ps_uvpcwd;
#ifdef DEBUG_UNVEIL
		printf("unveil: %s(%d): used cwd unveil vnode from vnode %p\n",
		    p->p_p->ps_comm, p->p_p->ps_pid, ni->ni_unveil_match->uv_vp);
#endif
		return (0);
	} else if (p->p_p->ps_uvpcwdgone) {
		printf("Corner cases make Bob cry in a corner\n");
	}
	return ENOENT;
}

/*
 * Scan all active processes to see if any of them have a unveil
 * to this vnode. If so, NULL the vnode in their unveil list,
 * vrele, drop the reference, and mark their unveil list
 * as needing to have the hole shrunk the next time the process
 * uses it for lookup.
 */
void
unveil_removevnode(struct vnode *vp)
{
	struct process *pr;

	if (vp->v_uvcount == 0)
		return;

#ifdef DEBUG_UNVEIL
	printf("unveil_removevnode found vnode %p with count %d\n",
	    vp, vp->v_uvcount);
#endif
	vref(vp); /* make sure it is held till we are done */

	LIST_FOREACH(pr, &allprocess, ps_list) {
		struct unveil * uv;

		if ((uv = unveil_lookup(vp, pr->ps_mainproc)) != NULL &&
		    uv->uv_vp != NULL) {
			uv->uv_vp = NULL;
			uv->uv_flags = 0;
#ifdef DEBUG_UNVEIL
			printf("unveil_removevnode vnode %p now count %d\n",
			    vp, vp->v_uvcount);
#endif
			pr->ps_uvshrink = 1;
			if (vp->v_uvcount > 0) {
				vrele(vp);
				vp->v_uvcount--;
			} else
				panic("vp %p, v_uvcount of %d should be 0",
				    vp, vp->v_uvcount);
		}
	}
	KASSERT(vp->v_uvcount == 0);

	vrele(vp); /* release our ref */
}