ovl: detect overlapping layers

Overlapping overlay layers are not supported and can cause unexpected
behavior, but overlayfs does not currently check or warn about these
configurations.

User is not supposed to specify the same directory for upper and
lower dirs or for different lower layers and user is not supposed to
specify directories that are descendants of each other for overlay
layers, but that is exactly what this zysbot repro did:

    https://syzkaller.appspot.com/x/repro.syz?x=12c7a94f400000

Moving layer root directories into other layers while overlayfs
is mounted could also result in unexpected behavior.

This commit places "traps" in the overlay inode hash table.
Those traps are dummy overlay inodes that are hashed by the layers
root inodes.

On mount, the hash table trap entries are used to verify that overlay
layers are not overlapping.  While at it, we also verify that overlay
layers are not overlapping with directories "in-use" by other overlay
instances as upperdir/workdir.

On lookup, the trap entries are used to verify that overlay layers
root inodes have not been moved into other layers after mount.

Some examples:

$ ./run --ov --samefs -s
...
( mkdir -p base/upper/0/u base/upper/0/w base/lower lower upper mnt
  mount -o bind base/lower lower
  mount -o bind base/upper upper
  mount -t overlay none mnt ...
        -o lowerdir=lower,upperdir=upper/0/u,workdir=upper/0/w)

$ umount mnt
$ mount -t overlay none mnt ...
        -o lowerdir=base,upperdir=upper/0/u,workdir=upper/0/w

  [   94.434900] overlayfs: overlapping upperdir path
  mount: mount overlay on mnt failed: Too many levels of symbolic links

$ mount -t overlay none mnt ...
        -o lowerdir=upper/0/u,upperdir=upper/0/u,workdir=upper/0/w

  [  151.350132] overlayfs: conflicting lowerdir path
  mount: none is already mounted or mnt busy

$ mount -t overlay none mnt ...
        -o lowerdir=lower:lower/a,upperdir=upper/0/u,workdir=upper/0/w

  [  201.205045] overlayfs: overlapping lowerdir path
  mount: mount overlay on mnt failed: Too many levels of symbolic links

$ mount -t overlay none mnt ...
        -o lowerdir=lower,upperdir=upper/0/u,workdir=upper/0/w
$ mv base/upper/0/ base/lower/
$ find mnt/0
  mnt/0
  mnt/0/w
  find: 'mnt/0/w/work': Too many levels of symbolic links
  find: 'mnt/0/u': Too many levels of symbolic links

Reported-by: syzbot+9c69c282adc4edd2b540@syzkaller.appspotmail.com
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

1 files changed, 152 insertions, 17 deletions

diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 5ec4fc2f5d7e..746ea36f3171 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -215,6 +215,9 @@ static void ovl_free_fs(struct ovl_fs *ofs)
 {
 	unsigned i;
 
+	iput(ofs->indexdir_trap);
+	iput(ofs->workdir_trap);
+	iput(ofs->upperdir_trap);
 	dput(ofs->indexdir);
 	dput(ofs->workdir);
 	if (ofs->workdir_locked)
@@ -223,8 +226,10 @@ static void ovl_free_fs(struct ovl_fs *ofs)
 	if (ofs->upperdir_locked)
 		ovl_inuse_unlock(ofs->upper_mnt->mnt_root);
 	mntput(ofs->upper_mnt);
-	for (i = 0; i < ofs->numlower; i++)
+	for (i = 0; i < ofs->numlower; i++) {
+		iput(ofs->lower_layers[i].trap);
 		mntput(ofs->lower_layers[i].mnt);
+	}
 	for (i = 0; i < ofs->numlowerfs; i++)
 		free_anon_bdev(ofs->lower_fs[i].pseudo_dev);
 	kfree(ofs->lower_layers);
@@ -983,7 +988,26 @@ static const struct xattr_handler *ovl_xattr_handlers[] = {
 	NULL
 };
 
-static int ovl_get_upper(struct ovl_fs *ofs, struct path *upperpath)
+static int ovl_setup_trap(struct super_block *sb, struct dentry *dir,
+			  struct inode **ptrap, const char *name)
+{
+	struct inode *trap;
+	int err;
+
+	trap = ovl_get_trap_inode(sb, dir);
+	err = PTR_ERR(trap);
+	if (IS_ERR(trap)) {
+		if (err == -ELOOP)
+			pr_err("overlayfs: conflicting %s path\n", name);
+		return err;
+	}
+
+	*ptrap = trap;
+	return 0;
+}
+
+static int ovl_get_upper(struct super_block *sb, struct ovl_fs *ofs,
+			 struct path *upperpath)
 {
 	struct vfsmount *upper_mnt;
 	int err;
@@ -1003,6 +1027,11 @@ static int ovl_get_upper(struct ovl_fs *ofs, struct path *upperpath)
 	if (err)
 		goto out;
 
+	err = ovl_setup_trap(sb, upperpath->dentry, &ofs->upperdir_trap,
+			     "upperdir");
+	if (err)
+		goto out;
+
 	upper_mnt = clone_private_mount(upperpath);
 	err = PTR_ERR(upper_mnt);
 	if (IS_ERR(upper_mnt)) {
@@ -1029,7 +1058,8 @@ out:
 	return err;
 }
 
-static int ovl_make_workdir(struct ovl_fs *ofs, struct path *workpath)
+static int ovl_make_workdir(struct super_block *sb, struct ovl_fs *ofs,
+			    struct path *workpath)
 {
 	struct vfsmount *mnt = ofs->upper_mnt;
 	struct dentry *temp;
@@ -1044,6 +1074,10 @@ static int ovl_make_workdir(struct ovl_fs *ofs, struct path *workpath)
 	if (!ofs->workdir)
 		goto out;
 
+	err = ovl_setup_trap(sb, ofs->workdir, &ofs->workdir_trap, "workdir");
+	if (err)
+		goto out;
+
 	/*
 	 * Upper should support d_type, else whiteouts are visible.  Given
 	 * workdir and upper are on same fs, we can do iterate_dir() on
@@ -1104,7 +1138,8 @@ out:
 	return err;
 }
 
-static int ovl_get_workdir(struct ovl_fs *ofs, struct path *upperpath)
+static int ovl_get_workdir(struct super_block *sb, struct ovl_fs *ofs,
+			   struct path *upperpath)
 {
 	int err;
 	struct path workpath = { };
@@ -1135,19 +1170,16 @@ static int ovl_get_workdir(struct ovl_fs *ofs, struct path *upperpath)
 		pr_warn("overlayfs: workdir is in-use by another mount, accessing files from both mounts will result in undefined behavior.\n");
 	}
 
-	err = ovl_make_workdir(ofs, &workpath);
-	if (err)
-		goto out;
+	err = ovl_make_workdir(sb, ofs, &workpath);
 
-	err = 0;
 out:
 	path_put(&workpath);
 
 	return err;
 }
 
-static int ovl_get_indexdir(struct ovl_fs *ofs, struct ovl_entry *oe,
-			    struct path *upperpath)
+static int ovl_get_indexdir(struct super_block *sb, struct ovl_fs *ofs,
+			    struct ovl_entry *oe, struct path *upperpath)
 {
 	struct vfsmount *mnt = ofs->upper_mnt;
 	int err;
@@ -1166,6 +1198,11 @@ static int ovl_get_indexdir(struct ovl_fs *ofs, struct ovl_entry *oe,
 
 	ofs->indexdir = ovl_workdir_create(ofs, OVL_INDEXDIR_NAME, true);
 	if (ofs->indexdir) {
+		err = ovl_setup_trap(sb, ofs->indexdir, &ofs->indexdir_trap,
+				     "indexdir");
+		if (err)
+			goto out;
+
 		/*
 		 * Verify upper root is exclusively associated with index dir.
 		 * Older kernels stored upper fh in "trusted.overlay.origin"
@@ -1253,8 +1290,8 @@ static int ovl_get_fsid(struct ovl_fs *ofs, const struct path *path)
 	return ofs->numlowerfs;
 }
 
-static int ovl_get_lower_layers(struct ovl_fs *ofs, struct path *stack,
-				unsigned int numlower)
+static int ovl_get_lower_layers(struct super_block *sb, struct ovl_fs *ofs,
+				struct path *stack, unsigned int numlower)
 {
 	int err;
 	unsigned int i;
@@ -1272,16 +1309,28 @@ static int ovl_get_lower_layers(struct ovl_fs *ofs, struct path *stack,
 
 	for (i = 0; i < numlower; i++) {
 		struct vfsmount *mnt;
+		struct inode *trap;
 		int fsid;
 
 		err = fsid = ovl_get_fsid(ofs, &stack[i]);
 		if (err < 0)
 			goto out;
 
+		err = -EBUSY;
+		if (ovl_is_inuse(stack[i].dentry)) {
+			pr_err("overlayfs: lowerdir is in-use as upperdir/workdir\n");
+			goto out;
+		}
+
+		err = ovl_setup_trap(sb, stack[i].dentry, &trap, "lowerdir");
+		if (err)
+			goto out;
+
 		mnt = clone_private_mount(&stack[i]);
 		err = PTR_ERR(mnt);
 		if (IS_ERR(mnt)) {
 			pr_err("overlayfs: failed to clone lowerpath\n");
+			iput(trap);
 			goto out;
 		}
 
@@ -1291,6 +1340,7 @@ static int ovl_get_lower_layers(struct ovl_fs *ofs, struct path *stack,
 		 */
 		mnt->mnt_flags |= MNT_READONLY | MNT_NOATIME;
 
+		ofs->lower_layers[ofs->numlower].trap = trap;
 		ofs->lower_layers[ofs->numlower].mnt = mnt;
 		ofs->lower_layers[ofs->numlower].idx = i + 1;
 		ofs->lower_layers[ofs->numlower].fsid = fsid;
@@ -1385,7 +1435,7 @@ static struct ovl_entry *ovl_get_lowerstack(struct super_block *sb,
 		goto out_err;
 	}
 
-	err = ovl_get_lower_layers(ofs, stack, numlower);
+	err = ovl_get_lower_layers(sb, ofs, stack, numlower);
 	if (err)
 		goto out_err;
 
@@ -1417,6 +1467,85 @@ out_err:
 	goto out;
 }
 
+/*
+ * Check if this layer root is a descendant of:
+ * - another layer of this overlayfs instance
+ * - upper/work dir of any overlayfs instance
+ * - a disconnected dentry (detached root)
+ */
+static int ovl_check_layer(struct super_block *sb, struct dentry *dentry,
+			   const char *name)
+{
+	struct dentry *next, *parent;
+	bool is_root = false;
+	int err = 0;
+
+	if (!dentry || dentry == dentry->d_sb->s_root)
+		return 0;
+
+	next = dget(dentry);
+	/* Walk back ancestors to fs root (inclusive) looking for traps */
+	do {
+		parent = dget_parent(next);
+		is_root = (parent == next);
+		if (ovl_is_inuse(parent)) {
+			err = -EBUSY;
+			pr_err("overlayfs: %s path overlapping in-use upperdir/workdir\n",
+			       name);
+		} else if (ovl_lookup_trap_inode(sb, parent)) {
+			err = -ELOOP;
+			pr_err("overlayfs: overlapping %s path\n", name);
+		}
+		dput(next);
+		next = parent;
+	} while (!err && !is_root);
+
+	/* Did we really walk to fs root or found a detached root? */
+	if (!err && next != dentry->d_sb->s_root) {
+		err = -ESTALE;
+		pr_err("overlayfs: disconnected %s path\n", name);
+	}
+
+	dput(next);
+
+	return err;
+}
+
+/*
+ * Check if any of the layers or work dirs overlap.
+ */
+static int ovl_check_overlapping_layers(struct super_block *sb,
+					struct ovl_fs *ofs)
+{
+	int i, err;
+
+	if (ofs->upper_mnt) {
+		err = ovl_check_layer(sb, ofs->upper_mnt->mnt_root, "upperdir");
+		if (err)
+			return err;
+
+		/*
+		 * Checking workbasedir avoids hitting ovl_is_inuse(parent) of
+		 * this instance and covers overlapping work and index dirs,
+		 * unless work or index dir have been moved since created inside
+		 * workbasedir.  In that case, we already have their traps in
+		 * inode cache and we will catch that case on lookup.
+		 */
+		err = ovl_check_layer(sb, ofs->workbasedir, "workdir");
+		if (err)
+			return err;
+	}
+
+	for (i = 0; i < ofs->numlower; i++) {
+		err = ovl_check_layer(sb, ofs->lower_layers[i].mnt->mnt_root,
+				      "lowerdir");
+		if (err)
+			return err;
+	}
+
+	return 0;
+}
+
 static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 {
 	struct path upperpath = { };
@@ -1456,17 +1585,20 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 	if (ofs->config.xino != OVL_XINO_OFF)
 		ofs->xino_bits = BITS_PER_LONG - 32;
 
+	/* alloc/destroy_inode needed for setting up traps in inode cache */
+	sb->s_op = &ovl_super_operations;
+
 	if (ofs->config.upperdir) {
 		if (!ofs->config.workdir) {
 			pr_err("overlayfs: missing 'workdir'\n");
 			goto out_err;
 		}
 
-		err = ovl_get_upper(ofs, &upperpath);
+		err = ovl_get_upper(sb, ofs, &upperpath);
 		if (err)
 			goto out_err;
 
-		err = ovl_get_workdir(ofs, &upperpath);
+		err = ovl_get_workdir(sb, ofs, &upperpath);
 		if (err)
 			goto out_err;
 
@@ -1487,7 +1619,7 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 		sb->s_flags |= SB_RDONLY;
 
 	if (!(ovl_force_readonly(ofs)) && ofs->config.index) {
-		err = ovl_get_indexdir(ofs, oe, &upperpath);
+		err = ovl_get_indexdir(sb, ofs, oe, &upperpath);
 		if (err)
 			goto out_free_oe;
 
@@ -1500,6 +1632,10 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 
 	}
 
+	err = ovl_check_overlapping_layers(sb, ofs);
+	if (err)
+		goto out_free_oe;
+
 	/* Show index=off in /proc/mounts for forced r/o mount */
 	if (!ofs->indexdir) {
 		ofs->config.index = false;
@@ -1521,7 +1657,6 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 	cap_lower(cred->cap_effective, CAP_SYS_RESOURCE);
 
 	sb->s_magic = OVERLAYFS_SUPER_MAGIC;
-	sb->s_op = &ovl_super_operations;
 	sb->s_xattr = ovl_xattr_handlers;
 	sb->s_fs_info = ofs;
 	sb->s_flags |= SB_POSIXACL;