linux-kernel - Re: [PATCH v2 1/3] x86/mm/KASLR: Fix the wrong calculation of kalsr region initial size

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Message-ID: <20180910061839.GA90334@gmail.com>
Date:   Mon, 10 Sep 2018 08:18:39 +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>
Subject: Re: [PATCH v2 1/3] x86/mm/KASLR: Fix the wrong calculation of kalsr
 region initial size


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

> In memory KASLR, __PHYSICAL_MASK_SHIFT is taken to calculate the
> initial size of the direct mapping region. This is right in the
> old code where __PHYSICAL_MASK_SHIFT was equal to MAX_PHYSMEM_BITS,
> 46bit, and only 4-level mode was supported.
> 
> Later, in commit:
> b83ce5ee91471d ("x86/mm/64: Make __PHYSICAL_MASK_SHIFT always 52"),
> __PHYSICAL_MASK_SHIFT was changed to be 52 always, no matter it's
> 5-level or 4-level. This is wrong for 4-level paging. Then when
> adapt phyiscal memory region size based on available memory, it
> will overflow if the amount of system RAM and the padding is bigger
> than 64TB.
> 
> In fact, here MAX_PHYSMEM_BITS should be used instead. Fix it by
> replacing __PHYSICAL_MASK_SHIFT with MAX_PHYSMEM_BITS.
> 
> Fixes: b83ce5ee9147 ("x86/mm/64: Make __PHYSICAL_MASK_SHIFT always 52")
> Signed-off-by: Baoquan He <bhe@...hat.com>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Reviewed-by: Thomas Garnier <thgarnie@...gle.com>

So this changelog has a handful of problems:

 - there's a typo in the title

 - what does 'memory KASLR' mean? All KASLR deals with memory.

 - there's a typo in the second paragraph

 - Please punctuate more precisely: '64TB' is written as '64 TB' and '46bit' is written as 
   '46 bits'

 - '52 always' is accurate but '52 bits always' would be more useful: write out units where
   appropriate to reduce  ambiguity and parsing complexity of changelogs. Also, in this
   particular sentence it should be 'always 52 bits'.

 - s/when adapt
    /when we adapt

 - s/This is right in the old code
    /This is correct in the old code

Thanks,

	Ingo