linux-kernel - Re: [PATCH 0/3] fs: reduce export usage of kerne

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Date:   Mon, 18 May 2020 08:37:42 -0400
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Christoph Hellwig <hch@...radead.org>,
        Luis Chamberlain <mcgrof@...nel.org>
Cc:     viro@...iv.linux.org.uk, gregkh@...uxfoundation.org,
        rafael@...nel.org, ebiederm@...ssion.com, jeyu@...nel.org,
        jmorris@...ei.org, keescook@...omium.org, paul@...l-moore.com,
        stephen.smalley.work@...il.com, eparis@...isplace.org,
        nayna@...ux.ibm.com, scott.branden@...adcom.com,
        dan.carpenter@...cle.com, skhan@...uxfoundation.org,
        geert@...ux-m68k.org, tglx@...utronix.de, bauerman@...ux.ibm.com,
        dhowells@...hat.com, linux-integrity@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, kexec@...ts.infradead.org,
        linux-security-module@...r.kernel.org, selinux@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] fs: reduce export usage of kerne_read*() calls

Hi Christoph,

On Sun, 2020-05-17 at 23:22 -0700, Christoph Hellwig wrote:
> On Fri, May 15, 2020 at 09:29:33PM +0000, Luis Chamberlain wrote:
> > On Wed, May 13, 2020 at 11:17:36AM -0700, Christoph Hellwig wrote:
> > > Can you also move kernel_read_* out of fs.h?  That header gets pulled
> > > in just about everywhere and doesn't really need function not related
> > > to the general fs interface.
> > 
> > Sure, where should I dump these?
> 
> Maybe a new linux/kernel_read_file.h?  Bonus points for a small top
> of the file comment explaining the point of the interface, which I
> still don't get :)

Instead of rolling your own method of having the kernel read a file,
which requires call specific security hooks, this interface provides a
single generic set of pre and post security hooks.  The
kernel_read_file_id enumeration permits the security hook to
differentiate between callers.

To comply with secure and trusted boot concepts, a file cannot be
accessible to the caller until after it has been measured and/or the
integrity (hash/signature) appraised.

In some cases, the file was previously read twice, first to measure
and/or appraise the file and then read again into a buffer for
use.  This interface reads the file into a buffer once, calls the
generic post security hook, before providing the buffer to the caller.
 (Note using firmware pre-allocated memory might be an issue.)

Partial reading firmware will result in needing to pre-read the entire
file, most likely on the security pre hook.

Mimi