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 16:18:11 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Ingo Molnar <mingo@...nel.org>
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

On 09/11/18 at 09:59am, Ingo Molnar wrote:
> 
> * 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?

* @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. Since most of systems own RAM memory
* which is much less than 64 TB which is reserved for mapping the maximum
* physical memory in 4-level paging mode, not to mention 5-level. The
* left space can be saved to enhance randomness.
* 
How about this? And please forgive my poor english.

Thanks
Baoquan