Messages in this thread | | | Date | Fri, 5 Apr 2019 19:22:46 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [PATCH 1/2] x86/mm/KASLR: Fix the wrong calculation of memory region initial size |
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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
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