mm, fs: move randomize_stack_top from fs to mm

Patch series "Provide generic top-down mmap layout functions", v6.

This series introduces generic functions to make top-down mmap layout
easily accessible to architectures, in particular riscv which was the
initial goal of this series.  The generic implementation was taken from
arm64 and used successively by arm, mips and finally riscv.

Note that in addition the series fixes 2 issues:

- stack randomization was taken into account even if not necessary.

- [1] fixed an issue with mmap base which did not take into account
  randomization but did not report it to arm and mips, so by moving arm64
  into a generic library, this problem is now fixed for both
  architectures.

This work is an effort to factorize architecture functions to avoid code
duplication and oversights as in [1].

[1]: https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1429066.html

This patch (of 14):

This preparatory commit moves this function so that further introduction
of generic topdown mmap layout is contained only in mm/util.c.

Link: http://lkml.kernel.org/r/20190730055113.23635-2-alex@ghiti.fr
Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Acked-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Paul Burton <paul.burton@mips.com>
Cc: James Hogan <jhogan@kernel.org>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 22 insertions, 0 deletions

diff --git a/mm/util.c b/mm/util.c
index 37f7b6711514..bf8af5e07c4a 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -16,6 +16,8 @@
 #include <linux/hugetlb.h>
 #include <linux/vmalloc.h>
 #include <linux/userfaultfd_k.h>
+#include <linux/elf.h>
+#include <linux/random.h>
 
 #include <linux/uaccess.h>
 
@@ -293,6 +295,26 @@ int vma_is_stack_for_current(struct vm_area_struct *vma)
 	return (vma->vm_start <= KSTK_ESP(t) && vma->vm_end >= KSTK_ESP(t));
 }
 
+#ifndef STACK_RND_MASK
+#define STACK_RND_MASK (0x7ff >> (PAGE_SHIFT - 12))     /* 8MB of VA */
+#endif
+
+unsigned long randomize_stack_top(unsigned long stack_top)
+{
+	unsigned long random_variable = 0;
+
+	if (current->flags & PF_RANDOMIZE) {
+		random_variable = get_random_long();
+		random_variable &= STACK_RND_MASK;
+		random_variable <<= PAGE_SHIFT;
+	}
+#ifdef CONFIG_STACK_GROWSUP
+	return PAGE_ALIGN(stack_top) + random_variable;
+#else
+	return PAGE_ALIGN(stack_top) - random_variable;
+#endif
+}
+
 #if defined(CONFIG_MMU) && !defined(HAVE_ARCH_PICK_MMAP_LAYOUT)
 void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
 {