linux-kernel - Re: [PATCH v2 2/3] x86/mm/KASLR: Calculate the actual size of vmemmap region

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Date:   Tue, 11 Sep 2018 09:59:46 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Baoquan He <bhe@...hat.com>
Cc:     tglx@...utronix.de, hpa@...or.com, thgarnie@...gle.com,
        kirill.shutemov@...ux.intel.com, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v2 2/3] x86/mm/KASLR: Calculate the actual size of
 vmemmap region

* Baoquan He <bhe@...hat.com> wrote:

>  /*
> + * Memory regions randomized by KASLR (except modules that use a separate
> + * logic earlier during boot). Currently they are the physical memory
> + * mapping, vmalloc and vmemmap regions, are ordered based on virtual
> + * addresses. The order is kept after randomization.
> + *
> + * @base: points to various global variables used by the MM to get the
> + * virtual base address of the above regions, which base addresses can
> + * thus be modified by the very early KASLR code to dynamically shape
> + * the virtual memory layout of these kernel memory regions on a per
> + * bootup basis.
> + *
> + * @size_tb: size in TB of each memory region. Thereinto, the size of
> + * the physical memory mapping region is variable, calculated according
> + * to the actual size of system RAM in order to save more space for
> + * randomization. The rest are fixed values related to paging mode.
>   */
>  static __initdata struct kaslr_memory_region {
>  	unsigned long *base;

LGTM mostly, except the @size_tb field, see my comments further below.

Here's an edited version:

/*
 * 'struct kasl_memory_region' entries represent continuous chunks of
 * kernel virtual memory regions, to be randomized by KASLR.
 *
 * ( The exception is the module space virtual memory window which
 *   uses separate logic earlier during bootup. )
 *
 * Currently there are three such regions: the physical memory mapping,
 * vmalloc and vmemmap regions.
 *
 * The array below has the entries ordered based on virtual addresses.
 * The order is kept after randomization, i.e. the randomized
 * virtual addresses of these regions are still ascending.
 *
 * Here are the fields:
 *
 * @base: points to a global variable used by the MM to get the
 * virtual base address of any of the above regions. This allows the
 * early KASLR code to modify these base addresses early during bootup,
 * on a per bootup basis, without the MM code even being aware of whether
 * it got changed and to what value.
 *
 * When KASLR is active then the MM code makes sure that for each region
 * there's such a single, dynamic, global base address 'unsigned long'
 * variable available for the KASLR code to point to and modify directly:
 *
 *       { &page_offset_base, 0 },
 *       { &vmalloc_base,     0 },
 *       { &vmemmap_base,     1 },
 *
 * @size_tb: size in TB of each memory region. Thereinto, the size of
 * the physical memory mapping region is variable, calculated according
 * to the actual size of system RAM in order to save more space for
 * randomization. The rest are fixed values related to paging mode.
 */

The role of @size_tb is still murky to me. What is it telling us?
Maximum virtual memory range to randomize into? Why does this depend
on system RAM at all - aren't these all virtual addresses in a 64-bit
(well, 48-bit or 56-bit) address ranges?

I could read the code to figure this out, but the comment should already
explain this and it doesn't.

Thanks,

	Ingo