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

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Date:   Fri, 22 May 2020 15:24:32 -0700
From:   Scott Branden <scott.branden@...adcom.com>
To:     Mimi Zohar <zohar@...ux.ibm.com>,
        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, 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 Mimi,

On 2020-05-18 5:37 a.m., 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.
The entire file may be very large and not fit into a buffer.
Hence one of the reasons for a partial read of the file.
For security purposes, you need to change your code to limit the amount
of data it reads into a buffer at one time to not consume or run out of 
much memory.
>
> Mimi
Scott