linux-kernel - Re: [PATCH v2 7/8] x86/kaslr: Clean up slot handling

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Date:   Tue, 28 Jul 2020 16:04:45 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Kees Cook <keescook@...omium.org>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 7/8] x86/kaslr: Clean up slot handling

On Tue, Jul 28, 2020 at 12:34:45PM -0700, Kees Cook wrote:
> On Mon, Jul 27, 2020 at 07:08:00PM -0400, Arvind Sankar wrote:
> > The number of slots and slot areas can be unsigned int, since on 64-bit,
> > the maximum amount of memory is 2^52, the minimum alignment is 2^21, so
> > the slot number cannot be greater than 2^31. The slot areas are limited
> > by MAX_SLOT_AREA, currently 100. Replace the type used for slot number,
> > which is currently a mix of int and unsigned long, with unsigned int
> > consistently.
> 
> I think it's good to standardize the type, but let's go to unsigned long
> then we don't have to think about this again in the future.

Ok, I can do that instead.

> 
> > Drop unnecessary check that number of slots is not zero in
> > store_slot_info, it's guaranteed to be at least 1 by the calculation.
> > 
> > Drop unnecessary alignment of image_size to CONFIG_PHYSICAL_ALIGN in
> > find_random_virt_addr, it cannot change the result: the largest valid
> > slot is the largest n that satisfies
> 
> I view all of these things as robustness checks. It doesn't hurt to do
> these checks and it'll avoid crashing into problems if future
> refactoring breaks assumptions.

Well, at least the first one should really be unnecessary: the previous
line sets it as 1 + x. When I see that it actually confuses me: I think
I must be missing some edge case where it could be zero.

The second one is also unnecessary, but I agree it might require a bit
of analysis to see that it is.

> > -	slots = (KERNEL_IMAGE_SIZE - minimum - image_size) /
> > -		 CONFIG_PHYSICAL_ALIGN + 1;
> > +	slots = 1 + (KERNEL_IMAGE_SIZE - minimum - image_size) / CONFIG_PHYSICAL_ALIGN;
> 
> These are the same -- why change the code?
> 

It's just reformatting now that we can have more than 80-column lines. I
think it's clearer this way, more obvious that you're dividing by
CONFIG_PHYSICAL_ALIGN and adding one, rather than "did he forget the
parentheses here".