fs: limit filesystem stacking depth

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@suse.cz>
[bwh: Backported to 3.2:
 - Drop changes to overlayfs
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/fs/ecryptfs/main.c b/fs/ecryptfs/main.c
index 62b8ddc2c630091ea87f71af826d94a959eeadb2..b53a97e43c91f794aab52c16fd102e379fdfafd8 100644 (file)
--- a/fs/ecryptfs/main.c
+++ b/fs/ecryptfs/main.c
@@ -576,6 +576,13 @@ 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);

diff --cc include/linux/fs.h
Simple merge