[PATCH 0/6] File Sealing & memfd_create()

[tytso at mit.edu]

On Wed, Mar 19, 2014 at 08:06:45PM +0100, David Herrmann wrote:
> 
> This series introduces the concept of "file sealing". Sealing a file restricts
> the set of allowed operations on the file in question. Multiple seals are
> defined and each seal will cause a different set of operations to return EPERM
> if it is set. The following seals are introduced:
> 
>  * SEAL_SHRINK: If set, the inode size cannot be reduced
>  * SEAL_GROW: If set, the inode size cannot be increased
>  * SEAL_WRITE: If set, the file content cannot be modified

Looking at your patches, and what files you are modifying, you are
enforcing this in the low-level file system.

Why not make sealing an attribute of the "struct file", and enforce it
at the VFS layer?  That way all file system objects would have access
to sealing interface, and for memfd_shmem, you can't get another
struct file pointing at the object, the security properties would be
identical.

Cheers,

						- Ted