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

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Date:   Tue, 28 Jul 2020 12:59:26 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Kees Cook <keescook@...omium.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 7/8] x86/kaslr: Clean up slot handling


* Arvind Sankar <nivedita@...m.mit.edu> 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.
> 
> 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
> 
>   minimum + n * CONFIG_PHYSICAL_ALIGN + image_size <= KERNEL_IMAGE_SIZE
> 
> (since minimum is already aligned) and so n is equal to
> 
>   (KERNEL_IMAGE_SIZE - minimum - image_size) / CONFIG_PHYSICAL_ALIGN
> 
> even if image_size is not aligned to CONFIG_PHYSICAL_ALIGN.

Please split this up into a couple of easily bisectable patches as 
well:

 - first one harmonizes the integer types used

 - patch that drops the unnecessary check

 - patch that drops the unnecessary alignment operation

Thanks,

	Ingo