Re: [PATCH 0/3] fs: reduce export usage of kerne_read*() calls

[Scott Branden]

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.ÂÂThe
> 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.ÂÂThis interface reads the file into a buffer once, calls the
> generic post security hook, before providing the buffer to the caller.
>  Â(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