arm64 handles top-down mmap layout in a way that can be easily reused
by other architectures, so make it available in mm.
It then introduces a new config ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
that can be set by other architectures to benefit from those functions.
Note that this new config depends on MMU being enabled, if selected
without MMU support, a warning will be thrown.

Suggested-by: Christoph Hellwig <h...@infradead.org>
Signed-off-by: Alexandre Ghiti <a...@ghiti.fr>
Acked-by: Catalin Marinas <catalin.mari...@arm.com>
Acked-by: Kees Cook <keesc...@chromium.org>
Reviewed-by: Christoph Hellwig <h...@lst.de>
Reviewed-by: Luis Chamberlain <mcg...@kernel.org>
---
 arch/Kconfig                       | 10 ++++
 arch/arm64/Kconfig                 |  1 +
 arch/arm64/include/asm/processor.h |  2 -
 arch/arm64/mm/mmap.c               | 76 -----------------------------
 kernel/sysctl.c                    |  6 ++-
 mm/util.c                          | 78 +++++++++++++++++++++++++++++-
 6 files changed, 92 insertions(+), 81 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index a7b57dd42c26..a0bb6fa4d381 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -696,6 +696,16 @@ config HAVE_ARCH_COMPAT_MMAP_BASES
          and vice-versa 32-bit applications to call 64-bit mmap().
          Required for applications doing different bitness syscalls.
 
+# This allows to use a set of generic functions to determine mmap base
+# address by giving priority to top-down scheme only if the process
+# is not in legacy mode (compat task, unlimited stack size or
+# sysctl_legacy_va_layout).
+# Architecture that selects this option can provide its own version of:
+# - STACK_RND_MASK
+config ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
+       bool
+       depends on MMU
+
 config HAVE_COPY_THREAD_TLS
        bool
        help
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 3adcec05b1f6..14a194e63458 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -72,6 +72,7 @@ config ARM64
        select ARCH_SUPPORTS_INT128 if GCC_VERSION >= 50000 || CC_IS_CLANG
        select ARCH_SUPPORTS_NUMA_BALANCING
        select ARCH_WANT_COMPAT_IPC_PARSE_VERSION if COMPAT
+       select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
        select ARCH_WANT_FRAME_POINTERS
        select ARCH_WANT_HUGE_PMD_SHARE if ARM64_4K_PAGES || (ARM64_16K_PAGES 
&& !ARM64_VA_BITS_36)
        select ARCH_HAS_UBSAN_SANITIZE_ALL
diff --git a/arch/arm64/include/asm/processor.h 
b/arch/arm64/include/asm/processor.h
index 844e2964b0f5..65e2de00913f 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h
@@ -281,8 +281,6 @@ static inline void spin_lock_prefetch(const void *ptr)
                     "nop") : : "p" (ptr));
 }
 
-#define HAVE_ARCH_PICK_MMAP_LAYOUT
-
 #endif
 
 extern unsigned long __ro_after_init signal_minsigstksz; /* sigframe size */
diff --git a/arch/arm64/mm/mmap.c b/arch/arm64/mm/mmap.c
index e4acaead67de..3028bacbc4e9 100644
--- a/arch/arm64/mm/mmap.c
+++ b/arch/arm64/mm/mmap.c
@@ -20,82 +20,6 @@
 
 #include <asm/cputype.h>
 
-/*
- * Leave enough space between the mmap area and the stack to honour ulimit in
- * the face of randomisation.
- */
-#define MIN_GAP (SZ_128M)
-#define MAX_GAP        (STACK_TOP/6*5)
-
-static int mmap_is_legacy(struct rlimit *rlim_stack)
-{
-       if (current->personality & ADDR_COMPAT_LAYOUT)
-               return 1;
-
-       if (rlim_stack->rlim_cur == RLIM_INFINITY)
-               return 1;
-
-       return sysctl_legacy_va_layout;
-}
-
-unsigned long arch_mmap_rnd(void)
-{
-       unsigned long rnd;
-
-#ifdef CONFIG_COMPAT
-       if (is_compat_task())
-               rnd = get_random_long() & ((1UL << mmap_rnd_compat_bits) - 1);
-       else
-#endif
-               rnd = get_random_long() & ((1UL << mmap_rnd_bits) - 1);
-       return rnd << PAGE_SHIFT;
-}
-
-static unsigned long mmap_base(unsigned long rnd, struct rlimit *rlim_stack)
-{
-       unsigned long gap = rlim_stack->rlim_cur;
-       unsigned long pad = stack_guard_gap;
-
-       /* Account for stack randomization if necessary */
-       if (current->flags & PF_RANDOMIZE)
-               pad += (STACK_RND_MASK << PAGE_SHIFT);
-
-       /* Values close to RLIM_INFINITY can overflow. */
-       if (gap + pad > gap)
-               gap += pad;
-
-       if (gap < MIN_GAP)
-               gap = MIN_GAP;
-       else if (gap > MAX_GAP)
-               gap = MAX_GAP;
-
-       return PAGE_ALIGN(STACK_TOP - gap - rnd);
-}
-
-/*
- * This function, called very early during the creation of a new process VM
- * image, sets up which VM layout function to use:
- */
-void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
-{
-       unsigned long random_factor = 0UL;
-
-       if (current->flags & PF_RANDOMIZE)
-               random_factor = arch_mmap_rnd();
-
-       /*
-        * Fall back to the standard layout if the personality bit is set, or
-        * if the expected stack growth is unlimited:
-        */
-       if (mmap_is_legacy(rlim_stack)) {
-               mm->mmap_base = TASK_UNMAPPED_BASE + random_factor;
-               mm->get_unmapped_area = arch_get_unmapped_area;
-       } else {
-               mm->mmap_base = mmap_base(random_factor, rlim_stack);
-               mm->get_unmapped_area = arch_get_unmapped_area_topdown;
-       }
-}
-
 /*
  * You really shouldn't be using read() or write() on /dev/mem.  This might go
  * away in the future.
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 078950d9605b..00fcea236eba 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -264,7 +264,8 @@ extern struct ctl_table epoll_table[];
 extern struct ctl_table firmware_config_table[];
 #endif
 
-#ifdef HAVE_ARCH_PICK_MMAP_LAYOUT
+#if defined(HAVE_ARCH_PICK_MMAP_LAYOUT) || \
+    defined(CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT)
 int sysctl_legacy_va_layout;
 #endif
 
@@ -1573,7 +1574,8 @@ static struct ctl_table vm_table[] = {
                .proc_handler   = proc_dointvec,
                .extra1         = SYSCTL_ZERO,
        },
-#ifdef HAVE_ARCH_PICK_MMAP_LAYOUT
+#if defined(HAVE_ARCH_PICK_MMAP_LAYOUT) || \
+    defined(CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT)
        {
                .procname       = "legacy_va_layout",
                .data           = &sysctl_legacy_va_layout,
diff --git a/mm/util.c b/mm/util.c
index 15a4fb0f5473..0781e5575cb3 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -17,7 +17,12 @@
 #include <linux/vmalloc.h>
 #include <linux/userfaultfd_k.h>
 #include <linux/elf.h>
+#include <linux/elf-randomize.h>
+#include <linux/personality.h>
 #include <linux/random.h>
+#include <linux/processor.h>
+#include <linux/sizes.h>
+#include <linux/compat.h>
 
 #include <linux/uaccess.h>
 
@@ -315,7 +320,78 @@ unsigned long randomize_stack_top(unsigned long stack_top)
 #endif
 }
 
-#if defined(CONFIG_MMU) && !defined(HAVE_ARCH_PICK_MMAP_LAYOUT)
+#ifdef CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
+#ifdef CONFIG_ARCH_HAS_ELF_RANDOMIZE
+unsigned long arch_mmap_rnd(void)
+{
+       unsigned long rnd;
+
+#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
+       if (is_compat_task())
+               rnd = get_random_long() & ((1UL << mmap_rnd_compat_bits) - 1);
+       else
+#endif /* CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS */
+               rnd = get_random_long() & ((1UL << mmap_rnd_bits) - 1);
+
+       return rnd << PAGE_SHIFT;
+}
+#endif /* CONFIG_ARCH_HAS_ELF_RANDOMIZE */
+
+static int mmap_is_legacy(struct rlimit *rlim_stack)
+{
+       if (current->personality & ADDR_COMPAT_LAYOUT)
+               return 1;
+
+       if (rlim_stack->rlim_cur == RLIM_INFINITY)
+               return 1;
+
+       return sysctl_legacy_va_layout;
+}
+
+/*
+ * Leave enough space between the mmap area and the stack to honour ulimit in
+ * the face of randomisation.
+ */
+#define MIN_GAP                (SZ_128M)
+#define MAX_GAP                (STACK_TOP / 6 * 5)
+
+static unsigned long mmap_base(unsigned long rnd, struct rlimit *rlim_stack)
+{
+       unsigned long gap = rlim_stack->rlim_cur;
+       unsigned long pad = stack_guard_gap;
+
+       /* Account for stack randomization if necessary */
+       if (current->flags & PF_RANDOMIZE)
+               pad += (STACK_RND_MASK << PAGE_SHIFT);
+
+       /* Values close to RLIM_INFINITY can overflow. */
+       if (gap + pad > gap)
+               gap += pad;
+
+       if (gap < MIN_GAP)
+               gap = MIN_GAP;
+       else if (gap > MAX_GAP)
+               gap = MAX_GAP;
+
+       return PAGE_ALIGN(STACK_TOP - gap - rnd);
+}
+
+void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
+{
+       unsigned long random_factor = 0UL;
+
+       if (current->flags & PF_RANDOMIZE)
+               random_factor = arch_mmap_rnd();
+
+       if (mmap_is_legacy(rlim_stack)) {
+               mm->mmap_base = TASK_UNMAPPED_BASE + random_factor;
+               mm->get_unmapped_area = arch_get_unmapped_area;
+       } else {
+               mm->mmap_base = mmap_base(random_factor, rlim_stack);
+               mm->get_unmapped_area = arch_get_unmapped_area_topdown;
+       }
+}
+#elif defined(CONFIG_MMU) && !defined(HAVE_ARCH_PICK_MMAP_LAYOUT)
 void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
 {
        mm->mmap_base = TASK_UNMAPPED_BASE;
-- 
2.20.1

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