Re: [PATCH 1/2] x86/mm/KASLR: Fix the wrong calculation of memory region initial size

From: Borislav Petkov
Date: Fri Apr 05 2019 - 12:59:09 EST

On Thu, Apr 04, 2019 at 10:03:13AM +0800, Baoquan He wrote:
> In memory region KASLR, __PHYSICAL_MASK_SHIFT is taken to calculate

What is "memory region KASLR"?

> the initial size of the direct mapping region. This is correct in
> the old code where __PHYSICAL_MASK_SHIFT was equal to MAX_PHYSMEM_BITS,
> 46 bits, 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 always 52 bits, no matter it's
> 5-level or 4-level.
> This is wrong for 4-level paging since it may cause randomness of KASLR
> being greatly weakened in 4-level. For KASLR, we compare the sum of RAM
> size and CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING with the size of the
> max RAM which can be supported by system, then choose the bigger one as
> the value to reserve space for the direct mapping region. The max RAM
> supported in 4-level is 64 TB according to MAX_PHYSMEM_BITS. However,
> here it's 4 PB in code to be compared with when __PHYSICAL_MASK_SHIFT is
> mistakenly used. E.g in a system owning 64 TB RAM, it will reserve 74 TB
> it should reserve 64 TB according to the algorithm which is supposed to
> do. Obviously the extra 10 TB space should be saved to join randomization.

It is not a trivial situation you're trying to explain and that
paragraph is very very hard to understand. I can only rhyme up what
you're trying to say.

So please rewrite it using simple declarative sentences. Don't try to
say three things in one sentence but say one thing in three sentences.
Keep it simple.



Good mailing practices for 400: avoid top-posting and trim the reply.