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

[Greg Kroah-Hartman <greg@kroah.com>]

On Wed, Mar 19, 2014 at 08:06:45PM +0100, David Herrmann wrote:
> Hi
> 
> 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
> 
> Unlike existing techniques that provide similar protection, sealing allows
> file-sharing without any trust-relationship. This is enforced by rejecting seal
> modifications if you don't own an exclusive reference to the given file. So if
> you own a file-descriptor, you can be sure that no-one besides you can modify
> the seals on the given file. This allows mapping shared files from untrusted
> parties without the fear of the file getting truncated or modified by an
> attacker.
> 
> Several use-cases exist that could make great use of sealing:
> 
>   1) Graphics Compositors
>      If a graphics client creates a memory-backed render-buffer and passes a
>      file-decsriptor to it to the graphics server for display, the server
>      _has_ to setup SIGBUS handlers whenever mapping the given file. Otherwise,
>      the client might run ftruncate() or O_TRUNC on the on file in parallel,
>      thus crashing the server.
>      With sealing, a compositor can reject any incoming file-descriptor that
>      does _not_ have SEAL_SHRINK set. This way, any memory-mappings are
>      guaranteed to stay accessible. Furthermore, we still allow clients to
>      increase the buffer-size in case they want to resize the render-buffer for
>      the next frame. We also allow parallel writes so the client can render new
>      frames into the same buffer (client is responsible of never rendering into
>      a front-buffer if you want to avoid artifacts).
> 
>      Real use-case: Wayland wl_shm buffers can be transparently converted

Very nice, the Enlightenment developers have been asking for something
like this for a while, it should help them out a lot as well.

And thanks for the man pages and test code, if only all new apis came
with that already...

greg k-h