Revert "mm: mmap: allow for the maximum number of bits for randomizing mmap_base by default"

BugLink: https://bugs.launchpad.net/bugs/2075154

commit 14d7c92 upstream.

This reverts commit 3afb76a.

This was a wrongheaded workaround for an issue that had already been
fixed much better by commit 4ef9ad1 ("mm: huge_memory: don't force
huge page alignment on 32 bit").

Asking users questions at kernel compile time that they can't make sense
of is not a viable strategy.  And the fact that even the kernel VM
maintainers apparently didn't catch that this "fix" is not a fix any
more pretty much proves the point that people can't be expected to
understand the implications of the question.

It may well be the case that we could improve things further, and that
__thp_get_unmapped_area() should take the mapping randomization into
account even for 64-bit kernels.  Maybe we should not be so eager to use
THP mappings.

But in no case should this be a kernel config option.

Cc: Rafael Aquini <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Jiri Slaby <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Manuel Diewald <[email protected]>
Signed-off-by: Stefan Bader <[email protected]>

Author: torvalds

arch/Kconfig

@@ -1029,21 +1029,10 @@ config ARCH_MMAP_RND_BITS_MAX
 config ARCH_MMAP_RND_BITS_DEFAULT
 	int
 
-config FORCE_MAX_MMAP_RND_BITS
-	bool "Force maximum number of bits to use for ASLR of mmap base address"
-	default y if !64BIT
-	help
-	  ARCH_MMAP_RND_BITS and ARCH_MMAP_RND_COMPAT_BITS represent the number
-	  of bits to use for ASLR and if no custom value is assigned (EXPERT)
-	  then the architecture's lower bound (minimum) value is assumed.
-	  This toggle changes that default assumption to assume the arch upper
-	  bound (maximum) value instead.
-
 config ARCH_MMAP_RND_BITS
 	int "Number of bits to use for ASLR of mmap base address" if EXPERT
 	range ARCH_MMAP_RND_BITS_MIN ARCH_MMAP_RND_BITS_MAX
 	default ARCH_MMAP_RND_BITS_DEFAULT if ARCH_MMAP_RND_BITS_DEFAULT
-	default ARCH_MMAP_RND_BITS_MAX if FORCE_MAX_MMAP_RND_BITS
 	default ARCH_MMAP_RND_BITS_MIN
 	depends on HAVE_ARCH_MMAP_RND_BITS
 	help
@@ -1078,7 +1067,6 @@ config ARCH_MMAP_RND_COMPAT_BITS
 	int "Number of bits to use for ASLR of mmap base address for compatible applications" if EXPERT
 	range ARCH_MMAP_RND_COMPAT_BITS_MIN ARCH_MMAP_RND_COMPAT_BITS_MAX
 	default ARCH_MMAP_RND_COMPAT_BITS_DEFAULT if ARCH_MMAP_RND_COMPAT_BITS_DEFAULT
-	default ARCH_MMAP_RND_COMPAT_BITS_MAX if FORCE_MAX_MMAP_RND_BITS
 	default ARCH_MMAP_RND_COMPAT_BITS_MIN
 	depends on HAVE_ARCH_MMAP_RND_COMPAT_BITS
 	help