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

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Message-ID: <202005180820.46CEF3C2@keescook>
Date:   Mon, 18 May 2020 08:21:08 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Mimi Zohar <zohar@...ux.ibm.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Luis Chamberlain <mcgrof@...nel.org>, viro@...iv.linux.org.uk,
        gregkh@...uxfoundation.org, rafael@...nel.org,
        ebiederm@...ssion.com, jeyu@...nel.org, jmorris@...ei.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

On Mon, May 18, 2020 at 08:37:42AM -0400, Mimi Zohar wrote:
> 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.

Well described! :)

-- 
Kees Cook