[RFC PATCH 6/6] shiftfs: support nested shiftfs mounts

[Seth Forshee <seth.forshee@canonical.com>]

shiftfs mounts cannot be nested for two reasons -- global
CAP_SYS_ADMIN is required to set up a mark mount, and a single
functional shiftfs mount meets the filesystem stacking depth
limit.

The CAP_SYS_ADMIN requirement can be relaxed. All of the kernel
ids in a mount must be within that mount's s_user_ns, so all that
is needed is CAP_SYS_ADMIN within that s_user_ns.

The stack depth issue can be worked around with a couple of
adjustments. First, a mark mount doesn't really need to count
against the stacking depth as it doesn't contribute to the call
stack depth during filesystem operations. Therefore the mount
over the mark mount only needs to count as one more than the
lower filesystems stack depth.

Second, when the lower mount is shiftfs we can just skip down to
that mount's lower filesystem and shift ids relative to that.
There is no reason to shift ids twice, and the lower path has
already been marked safe for id shifting by a user privileged
towards all ids in that mount's user ns.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
---
 fs/shiftfs.c | 68 +++++++++++++++++++++++++++++++++++-----------------
 1 file changed, 46 insertions(+), 22 deletions(-)

diff --git a/fs/shiftfs.c b/fs/shiftfs.c
index b19af7b2fe75..008ace2842b9 100644
--- a/fs/shiftfs.c
+++ b/fs/shiftfs.c
@@ -930,7 +930,7 @@ static int shiftfs_fill_super(struct super_block *sb, void *raw_data,
 	struct shiftfs_data *data = raw_data;
 	char *name = kstrdup(data->path, GFP_KERNEL);
 	int err = -ENOMEM;
-	struct shiftfs_super_info *ssi = NULL;
+	struct shiftfs_super_info *ssi = NULL, *mp_ssi;
 	struct path path;
 	struct dentry *dentry;
 
@@ -946,11 +946,7 @@ static int shiftfs_fill_super(struct super_block *sb, void *raw_data,
 	if (err)
 		goto out;
 
-	/* to mark a mount point, must be real root */
-	if (ssi->mark && !capable(CAP_SYS_ADMIN))
-		goto out;
-
-	/* else to mount a mark, must be userns admin */
+	/* to mount a mark, must be userns admin */
 	if (!ssi->mark && !ns_capable(current_user_ns(), CAP_SYS_ADMIN))
 		goto out;
 
@@ -962,41 +958,66 @@ static int shiftfs_fill_super(struct super_block *sb, void *raw_data,
 
 	if (!S_ISDIR(path.dentry->d_inode->i_mode)) {
 		err = -ENOTDIR;
-		goto out_put;
-	}
-
-	sb->s_stack_depth = path.dentry->d_sb->s_stack_depth + 1;
-	if (sb->s_stack_depth > FILESYSTEM_MAX_STACK_DEPTH) {
-		printk(KERN_ERR "shiftfs: maximum stacking depth exceeded\n");
-		err = -EINVAL;
-		goto out_put;
+		goto out_put_path;
 	}
 
 	if (ssi->mark) {
+		struct super_block *lower_sb = path.mnt->mnt_sb;
+
+		/* to mark a mount point, must root wrt lower s_user_ns */
+		if (!ns_capable(lower_sb->s_user_ns, CAP_SYS_ADMIN))
+			goto out_put_path;
+
+
 		/*
 		 * this part is visible unshifted, so make sure no
 		 * executables that could be used to give suid
 		 * privileges
 		 */
 		sb->s_iflags = SB_I_NOEXEC;
-		ssi->mnt = path.mnt;
-		dentry = path.dentry;
-	} else {
-		struct shiftfs_super_info *mp_ssi;
 
+		/*
+		 * Handle nesting of shiftfs mounts by referring this mark
+		 * mount back to the original mark mount. This is more
+		 * efficient and alleviates concerns about stack depth.
+		 */
+		if (lower_sb->s_magic == SHIFTFS_MAGIC) {
+			mp_ssi = lower_sb->s_fs_info;
+
+			/* Doesn't make sense to mark a mark mount */
+			if (mp_ssi->mark) {
+				err = -EINVAL;
+				goto out_put_path;
+			}
+
+			ssi->mnt = mntget(mp_ssi->mnt);
+			dentry = dget(path.dentry->d_fsdata);
+		} else {
+			ssi->mnt = mntget(path.mnt);
+			dentry = dget(path.dentry);
+		}
+	} else {
 		/*
 		 * this leg executes if we're admin capable in
 		 * the namespace, so be very careful
 		 */
 		if (path.dentry->d_sb->s_magic != SHIFTFS_MAGIC)
-			goto out_put;
+			goto out_put_path;
 		mp_ssi = path.dentry->d_sb->s_fs_info;
 		if (!mp_ssi->mark)
-			goto out_put;
+			goto out_put_path;
 		ssi->mnt = mntget(mp_ssi->mnt);
 		dentry = dget(path.dentry->d_fsdata);
-		path_put(&path);
 	}
+
+	sb->s_stack_depth = dentry->d_sb->s_stack_depth + 1;
+	if (sb->s_stack_depth > FILESYSTEM_MAX_STACK_DEPTH) {
+		printk(KERN_ERR "shiftfs: maximum stacking depth exceeded\n");
+		err = -EINVAL;
+		goto out_put_mnt;
+	}
+
+	path_put(&path);
 	ssi->userns = get_user_ns(dentry->d_sb->s_user_ns);
 	sb->s_fs_info = ssi;
 	sb->s_magic = SHIFTFS_MAGIC;
@@ -1009,7 +1030,10 @@ static int shiftfs_fill_super(struct super_block *sb, void *raw_data,
 
 	return 0;
 
- out_put:
+ out_put_mnt:
+	mntput(ssi->mnt);
+	dput(dentry);
+ out_put_path:
 	path_put(&path);
  out:
 	kfree(name);
-- 
2.19.1