mmap_min_addr/SECURITY_DEFAULT_MMAP_MIN_ADDR suggested values

["Dr. David Alan Gilbert" <linux@treblig.org>]

Hi,
  The config help for SECURITY_DEFAULT_MMAP_MIN_ADDR suggests 65536
as a 'reasonable' value for x86 and the original mmap_min_addr
patches suggested that 'something like 64k' was a safe value that
wouldn't affect most programs.

  Where does this 64k value come from?  A number of distros seem
to have followed this advice and have it set to 64k; but is there
really any likely benefit of having it larger than PAGE_SIZE say?

I ask because I have an ancient program that maps stuff at 8k; the
general advice of stuff on the net seems to be to set this limit
to 0 if people have problems (I'm just lowering it to 4k),
but I was thinking perhaps using a lower default value would be more
secure since less people would take the easy answer and just turn the
feature off altogether.

Dave

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