fs: limit filesystem stacking depth

Add a simple read-only counter to super_block that indicates how deep this
is in the stack of filesystems.  Previously ecryptfs was the only stackable
filesystem and it explicitly disallowed multiple layers of itself.

Overlayfs, however, can be stacked recursively and also may be stacked
on top of ecryptfs or vice versa.

To limit the kernel stack usage we must limit the depth of the
filesystem stack.  Initially the limit is set to 2.
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

fs/ecryptfs/main.c

@@ -566,6 +566,13 @@ static struct dentry *ecryptfs_mount(struct file_system_type *fs_type, int flags
 	s->s_maxbytes = path.dentry->d_sb->s_maxbytes;
 	s->s_blocksize = path.dentry->d_sb->s_blocksize;
 	s->s_magic = ECRYPTFS_SUPER_MAGIC;
+	s->s_stack_depth = path.dentry->d_sb->s_stack_depth + 1;
+
+	rc = -EINVAL;
+	if (s->s_stack_depth > FILESYSTEM_MAX_STACK_DEPTH) {
+		pr_err("eCryptfs: maximum fs stacking depth exceeded\n");
+		goto out_free;
+	}
 
 	inode = ecryptfs_get_inode(path.dentry->d_inode, s);
 	rc = PTR_ERR(inode);

fs/overlayfs/super.c

@@ -677,6 +677,15 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 	}
 	ufs->lower_namelen = statfs.f_namelen;
 
+	sb->s_stack_depth = max(upperpath.mnt->mnt_sb->s_stack_depth,
+				lowerpath.mnt->mnt_sb->s_stack_depth) + 1;
+
+	err = -EINVAL;
+	if (sb->s_stack_depth > FILESYSTEM_MAX_STACK_DEPTH) {
+		pr_err("overlayfs: maximum fs stacking depth exceeded\n");
+		goto out_put_workpath;
+	}
+
 	ufs->upper_mnt = clone_private_mount(&upperpath);
 	err = PTR_ERR(ufs->upper_mnt);
 	if (IS_ERR(ufs->upper_mnt)) {

include/linux/fs.h

@@ -261,6 +261,12 @@ struct iattr {
  */
 #include <linux/quota.h>
 
+/*
+ * Maximum number of layers of fs stack.  Needs to be limited to
+ * prevent kernel stack overflow
+ */
+#define FILESYSTEM_MAX_STACK_DEPTH 2
+
 /** 
  * enum positive_aop_returns - aop return codes with specific semantics
  *
@@ -1273,6 +1279,11 @@ struct super_block {
 	struct list_lru		s_dentry_lru ____cacheline_aligned_in_smp;
 	struct list_lru		s_inode_lru ____cacheline_aligned_in_smp;
 	struct rcu_head		rcu;
+
+	/*
+	 * Indicates how deep in a filesystem stack this SB is
+	 */
+	int s_stack_depth;
 };
 
 extern struct timespec current_fs_time(struct super_block *sb);