arm64, mm: make randomization selected by generic topdown mmap layout

This commits selects ARCH_HAS_ELF_RANDOMIZE when an arch uses the generic
topdown mmap layout functions so that this security feature is on by
default.

Note that this commit also removes the possibility for arm64 to have elf
randomization and no MMU: without MMU, the security added by randomization
is worth nothing.

Link: http://lkml.kernel.org/r/20190730055113.23635-6-alex@ghiti.fr
Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Paul Burton <paul.burton@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/Kconfig

@@ -715,6 +715,7 @@ config HAVE_ARCH_COMPAT_MMAP_BASES
 config ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
 	bool
 	depends on MMU
+	select ARCH_HAS_ELF_RANDOMIZE
 
 config HAVE_COPY_THREAD_TLS
 	bool

arch/arm64/Kconfig

@@ -15,7 +15,6 @@ config ARM64
 	select ARCH_HAS_DMA_COHERENT_TO_PFN
 	select ARCH_HAS_DMA_PREP_COHERENT
 	select ARCH_HAS_ACPI_TABLE_UPGRADE if ACPI
-	select ARCH_HAS_ELF_RANDOMIZE
 	select ARCH_HAS_FAST_MULTIPLIER
 	select ARCH_HAS_FORTIFY_SOURCE
 	select ARCH_HAS_GCOV_PROFILE_ALL

arch/arm64/kernel/process.c

@@ -557,14 +557,6 @@ unsigned long arch_align_stack(unsigned long sp)
 	return sp & ~0xf;
 }
 
-unsigned long arch_randomize_brk(struct mm_struct *mm)
-{
-	if (is_compat_task())
-		return randomize_page(mm->brk, SZ_32M);
-	else
-		return randomize_page(mm->brk, SZ_1G);
-}
-
 /*
  * Called from setup_new_exec() after (COMPAT_)SET_PERSONALITY.
  */

mm/util.c

@@ -321,7 +321,15 @@ unsigned long randomize_stack_top(unsigned long stack_top)
 }
 
 #ifdef CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
-#ifdef CONFIG_ARCH_HAS_ELF_RANDOMIZE
+unsigned long arch_randomize_brk(struct mm_struct *mm)
+{
+	/* Is the current task 32bit ? */
+	if (!IS_ENABLED(CONFIG_64BIT) || is_compat_task())
+		return randomize_page(mm->brk, SZ_32M);
+
+	return randomize_page(mm->brk, SZ_1G);
+}
+
 unsigned long arch_mmap_rnd(void)
 {
 	unsigned long rnd;
@@ -335,7 +343,6 @@ unsigned long arch_mmap_rnd(void)
 
 	return rnd << PAGE_SHIFT;
 }
-#endif /* CONFIG_ARCH_HAS_ELF_RANDOMIZE */
 
 static int mmap_is_legacy(struct rlimit *rlim_stack)
 {