[PATCH 3.2 85/94] fs: limit filesystem stacking depth

[Ben Hutchings]

3.2.82-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Miklos Szeredi <mszeredi@xxxxxxx>

commit 69c433ed2ecd2d3264efd7afec4439524b319121 upstream.

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@xxxxxxx>
[bwh: Backported to 3.2:
 - Drop changes to overlayfs
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 fs/ecryptfs/main.c   |  7 +++++++
 include/linux/fs.h   | 11 +++++++++++
 3 files changed, 27 insertions(+)

--- a/fs/ecryptfs/main.c
+++ b/fs/ecryptfs/main.c
@@ -576,6 +576,13 @@ static struct dentry *ecryptfs_mount(str
 	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);
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -486,6 +486,12 @@ 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
  *
@@ -1497,6 +1503,11 @@ struct super_block {
 	int cleancache_poolid;
 
 	struct shrinker s_shrink;	/* per-sb shrinker handle */
+
+	/*
+	 * Indicates how deep in a filesystem stack this SB is
+	 */
+	int s_stack_depth;
 };
 
 /* superblock cache pruning functions */