The change in r224615 didn't take into account that vn_fullpath_global()

doesn't operate on locked vnode. This could cause a panic.

Fix by unlocking vnode, re-locking afterwards and verifying that it wasn't
renamed or deleted. To improve readability and reduce code size, move code
to a new static function vfs_verify_global_path().

In addition, fix missing giant unlock in unmount().

Reported by:	David Wolfskill <david@catwhisker.org>
Reviewed by:	kib
Approved by:	re (bz)
MFC after:	2 weeks

sys/kern/vfs_mount.c

@@ -361,6 +361,60 @@ vfs_mergeopts(struct vfsoptlist *toopts, struct vfsoptlist *oldopts)
 	vfs_sanitizeopts(toopts);
 }
 
+/*
+ * Verify vnode's global path
+ */
+static int
+vfs_verify_global_path(struct thread *td, struct vnode *vp, char *fspath)
+{
+	struct nameidata nd;
+	struct vnode *vp1;
+	char *rpath, *fbuf;
+	int error;
+
+	ASSERT_VOP_ELOCKED(vp, __func__);
+
+	/* Construct global filesystem path from vp. */
+	VOP_UNLOCK(vp, 0);
+	error = vn_fullpath_global(td, vp, &rpath, &fbuf);
+	if (error != 0) {
+		vrele(vp);
+		return (error);
+	}
+	if (strlen(rpath) >= MNAMELEN) {
+		vrele(vp);
+		error = ENAMETOOLONG;
+		goto out;
+	}
+
+	/*
+	 * Re-lookup the vnode by path. As a side effect, the vnode is
+	 * relocked.  If vnode was renamed, return ENOENT.
+	 */
+	NDINIT(&nd, LOOKUP, FOLLOW | LOCKLEAF | MPSAFE | AUDITVNODE1,
+	    UIO_SYSSPACE, fspath, td);
+	error = namei(&nd);
+	if (error != 0) {
+		vrele(vp);
+		goto out;
+	}
+	if (NDHASGIANT(&nd))
+		mtx_unlock(&Giant);
+	NDFREE(&nd, NDF_ONLY_PNBUF);
+	vp1 = nd.ni_vp;
+	vrele(vp);
+	if (vp1 != vp) {
+		vput(vp1);
+		error = ENOENT;
+		goto out;
+	}
+
+	strlcpy(fspath,rpath,MNAMELEN);
+out:
+	free(fbuf, M_TEMP);
+	return (error);
+}
+
 /*
  * Mount a filesystem.
  */
@@ -745,6 +799,7 @@ static int
 vfs_domount_first(
 	struct thread *td,		/* Calling thread. */
 	struct vfsconf *vfsp,		/* File system type. */
+	char *fspath,			/* Mount path. */
 	struct vnode *vp,		/* Vnode to be covered. */
 	int fsflags,			/* Flags common to all filesystems. */
 	struct vfsoptlist **optlist	/* Options local to the filesystem. */
@@ -753,25 +808,12 @@ vfs_domount_first(
 	struct vattr va;
 	struct mount *mp;
 	struct vnode *newdp;
-	char *fspath, *fbuf;
 	int error;
 
 	mtx_assert(&Giant, MA_OWNED);
 	ASSERT_VOP_ELOCKED(vp, __func__);
 	KASSERT((fsflags & MNT_UPDATE) == 0, ("MNT_UPDATE shouldn't be here"));
 
-	/* Construct global filesystem path from vp. */
-	error = vn_fullpath_global(td, vp, &fspath, &fbuf);
-	if (error != 0) {
-		vput(vp);
-		return (error);
-	}
-	if (strlen(fspath) >= MNAMELEN) {
-		vput(vp);
-		free(fbuf, M_TEMP);
-		return (ENAMETOOLONG);
-	}
-
 	/*
 	 * If the user is not root, ensure that they own the directory
 	 * onto which we are attempting to mount.
@@ -793,14 +835,12 @@ vfs_domount_first(
 	}
 	if (error != 0) {
 		vput(vp);
-		free(fbuf, M_TEMP);
 		return (error);
 	}
 	VOP_UNLOCK(vp, 0);
 
 	/* Allocate and initialize the filesystem. */
 	mp = vfs_mount_alloc(vp, vfsp, fspath, td->td_ucred);
-	free(fbuf, M_TEMP);
 	/* XXXMAC: pass to vfs_mount_alloc? */
 	mp->mnt_optnew = *optlist;
 	/* Set the mount level flags. */
@@ -1083,15 +1123,15 @@ vfs_domount(
 		mtx_lock(&Giant);
 	NDFREE(&nd, NDF_ONLY_PNBUF);
 	vp = nd.ni_vp;
-	if ((fsflags & MNT_UPDATE) == 0)
+	if ((fsflags & MNT_UPDATE) == 0) {
-		error = vfs_domount_first(td, vfsp, vp, fsflags, optlist);
+		error = vfs_verify_global_path(td, vp, fspath);
-	else
+		if (error == 0)
+			error = vfs_domount_first(td, vfsp, fspath, vp,
+			    fsflags, optlist);
+	} else
 		error = vfs_domount_update(td, vp, fsflags, optlist);
 	mtx_unlock(&Giant);
 
-	ASSERT_VI_UNLOCKED(vp, __func__);
-	ASSERT_VOP_UNLOCKED(vp, __func__);
-
 	return (error);
 }
 
@@ -1118,7 +1158,7 @@ unmount(td, uap)
 {
 	struct mount *mp;
 	struct nameidata nd;
-	char *pathbuf, *rpathbuf, *fbuf;
+	char *pathbuf;
 	int error, id0, id1;
 
 	AUDIT_ARG_VALUE(uap->flags);
@@ -1163,16 +1203,13 @@ unmount(td, uap)
 			    FOLLOW | LOCKLEAF | MPSAFE | AUDITVNODE1,
 			    UIO_SYSSPACE, pathbuf, td);
 			if (namei(&nd) == 0) {
+				if (NDHASGIANT(&nd))
+					mtx_unlock(&Giant);
 				NDFREE(&nd, NDF_ONLY_PNBUF);
-				if (vn_fullpath_global(td, nd.ni_vp, &rpathbuf,
+				error = vfs_verify_global_path(td, nd.ni_vp,
-				    &fbuf) == 0) {
+				    pathbuf);
-					if (strlen(rpathbuf) < MNAMELEN) {
+				if (error == 0)
-						strlcpy(pathbuf, rpathbuf,
+					vput(nd.ni_vp);
-						    MNAMELEN);
-					}
-					free(fbuf, M_TEMP);
-				}
-				vput(nd.ni_vp);
 			}
 		}
 		mtx_lock(&mountlist_mtx);