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

[Thomas Gleixner]

On Fri, 5 Apr 2019, Borislav Petkov wrote:
> 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
> > (which is 64 TB plus CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING). In fact
> > 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.

For complex scenarios a simple ascii scheme is often helpful

Situation A

    ------- LIMIT1

    ------- LIMIT2
			<- unused area
    -------

    ------- 0

Situation B

    ------- LIMIT1


		
    ------- LIMIT2

    ------- 0


I was not trying to depict your problem, it's just a random thing, but you get
the idea.

Thanks,

	tglx