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

[Scott Branden]

Hi Kees,

On 2020-05-22 4:04 p.m., Kees Cook wrote:
> On Fri, May 22, 2020 at 03:24:32PM -0700, Scott Branden wrote:
> > On 2020-05-18 5:37 a.m., Mimi Zohar wrote:
> > > 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.
> Hm? That's not how whole-file hashing works. :)

> These hooks need to finish their hashing and policy checking before they
> can allow the rest of the code to move forward. (That's why it's a
> security hook.) If kernel memory utilization is the primary concern,
> then sure, things could be rearranged to do partial read and update the
> hash incrementally, but the entire file still needs to be locked,
> entirely hashed by hook, then read by the caller, then unlocked and
> released.
>
> So, if you want to have partial file reads work, you'll need to
> rearchitect the way this works to avoid regressing the security coverage
> of these operations.
I am not familiar with how the security handling code works at all.
Is the same security check run on files opened from user space?
A file could be huge.

If it assumes there is there is enough memory available to read the 
entire file into kernel space
then the improvement below can be left as a memory optimization to be 
done in
an independent (or future) patch series.

> So, probably, the code will look something like:
>
>
> file = kernel_open_file_for_reading(...)
> 	file = open...
> 	disallow_writes(file);
> 	while (processed < size-of-file) {
> 		buf = read(file, size...)
> 		security_file_read_partial(buf)
> 	}
> 	ret = security_file_read_finished(file);
> 	if (ret < 0) {
> 		allow_writes(file);
> 		return PTR_ERR(ret);
> 	}
> 	return file;
>
> while (processed < size-of-file) {
> 	buf = read(file, size...)
> 	firmware_send_partial(buf);
> }
>
> kernel_close_file_for_reading(file)
> 	allow_writes(file);