[PATCH 6/8] mm: Allow arch specific arch_randomize_brk() with CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT

[Christophe Leroy]

Commit e7142bf5d231 ("arm64, mm: make randomization selected by
generic topdown mmap layout") introduced a default version of
arch_randomize_brk() provided when
CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT is selected.

powerpc could select CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
but needs to provide its own arch_randomize_brk().

In order to allow that, don't make
CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT select
CONFIG_ARCH_HAS_ELF_RANDOMIZE. Instead, ensure that
selecting CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT and
selecting CONFIG_ARCH_HAS_ELF_RANDOMIZE has the same effect.

Then only provide the default arch_randomize_brk() when the
architecture has not selected CONFIG_ARCH_HAS_ELF_RANDOMIZE.

Cc: Alexandre Ghiti <alex@xxxxxxxx>
Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
---
 arch/Kconfig                  | 1 -
 fs/binfmt_elf.c               | 3 ++-
 include/linux/elf-randomize.h | 3 ++-
 mm/util.c                     | 2 ++
 4 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 26b8ed11639d..ef3ce947b7a1 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -1000,7 +1000,6 @@ config HAVE_ARCH_COMPAT_MMAP_BASES
 config ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
 	bool
 	depends on MMU
-	select ARCH_HAS_ELF_RANDOMIZE
 
 config HAVE_STACK_VALIDATION
 	bool
diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index f8c7f26f1fbb..28968a189a91 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -1287,7 +1287,8 @@ static int load_elf_binary(struct linux_binprm *bprm)
 		 * (since it grows up, and may collide early with the stack
 		 * growing down), and into the unused ELF_ET_DYN_BASE region.
 		 */
-		if (IS_ENABLED(CONFIG_ARCH_HAS_ELF_RANDOMIZE) &&
+		if ((IS_ENABLED(CONFIG_ARCH_HAS_ELF_RANDOMIZE) ||
+		     IS_ENABLED(CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT)) &&
 		    elf_ex->e_type == ET_DYN && !interpreter) {
 			mm->brk = mm->start_brk = ELF_ET_DYN_BASE;
 		}
diff --git a/include/linux/elf-randomize.h b/include/linux/elf-randomize.h
index da0dbb7b6be3..1e471ca7caaf 100644
--- a/include/linux/elf-randomize.h
+++ b/include/linux/elf-randomize.h
@@ -4,7 +4,8 @@
 
 struct mm_struct;
 
-#ifndef CONFIG_ARCH_HAS_ELF_RANDOMIZE
+#if !defined(CONFIG_ARCH_HAS_ELF_RANDOMIZE) && \
+	!defined(CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT)
 static inline unsigned long arch_mmap_rnd(void) { return 0; }
 # if defined(arch_randomize_brk) && defined(CONFIG_COMPAT_BRK)
 #  define compat_brk_randomized
diff --git a/mm/util.c b/mm/util.c
index e58151a61255..edb9e94cceb5 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -344,6 +344,7 @@ unsigned long randomize_stack_top(unsigned long stack_top)
 }
 
 #ifdef CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
+#ifndef CONFIG_ARCH_HAS_ELF_RANDOMIZE
 unsigned long arch_randomize_brk(struct mm_struct *mm)
 {
 	/* Is the current task 32bit ? */
@@ -352,6 +353,7 @@ unsigned long arch_randomize_brk(struct mm_struct *mm)
 
 	return randomize_page(mm->brk, SZ_1G);
 }
+#endif
 
 unsigned long arch_mmap_rnd(void)
 {
-- 
2.33.1