Messages in this thread | | | Date | Sat, 6 Apr 2019 09:55:47 +0800 | From | Baoquan He <> | Subject | Re: [PATCH 1/2] x86/mm/KASLR: Fix the wrong calculation of memory region initial size |
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On 04/05/19 at 07:22pm, Thomas Gleixner wrote: > 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.
OK, got it. Will rewrite with simpler sentences, and some more understandable ways to depict. Thanks a lot.
Thanks Baoquan
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