x86: standardize mmap_rnd() usage

In preparation for splitting out ET_DYN ASLR, this refactors the use of
mmap_rnd() to be used similarly to arm, and extracts the checking of
PF_RANDOMIZE.

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 20 insertions, 16 deletions

diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
index df4552bd239e..ebfa52030d5c 100644
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c
@@ -67,22 +67,21 @@ static int mmap_is_legacy(void)
 
 static unsigned long mmap_rnd(void)
 {
-        unsigned long rnd = 0;
+        unsigned long rnd;
 
         /*
-        *  8 bits of randomness in 32bit mmaps, 20 address space bits
-        * 28 bits of randomness in 64bit mmaps, 40 address space bits
-        */
-        if (current->flags & PF_RANDOMIZE) {
-                if (mmap_is_ia32())
-                        rnd = get_random_int() % (1<<8);
-                else
-                        rnd = get_random_int() % (1<<28);
-        }
+         *  8 bits of randomness in 32bit mmaps, 20 address space bits
+         * 28 bits of randomness in 64bit mmaps, 40 address space bits
+         */
+        if (mmap_is_ia32())
+                rnd = (unsigned long)get_random_int() % (1<<8);
+        else
+                rnd = (unsigned long)get_random_int() % (1<<28);
+
         return rnd << PAGE_SHIFT;
 }
 
-static unsigned long mmap_base(void)
+static unsigned long mmap_base(unsigned long rnd)
 {
         unsigned long gap = rlimit(RLIMIT_STACK);
 
@@ -91,19 +90,19 @@ static unsigned long mmap_base(void)
         else if (gap > MAX_GAP)
                 gap = MAX_GAP;
 
-        return PAGE_ALIGN(TASK_SIZE - gap - mmap_rnd());
+        return PAGE_ALIGN(TASK_SIZE - gap - rnd);
 }
 
 /*
  * Bottom-up (legacy) layout on X86_32 did not support randomization, X86_64
  * does, but not when emulating X86_32
  */
-static unsigned long mmap_legacy_base(void)
+static unsigned long mmap_legacy_base(unsigned long rnd)
 {
         if (mmap_is_ia32())
                 return TASK_UNMAPPED_BASE;
         else
-                return TASK_UNMAPPED_BASE + mmap_rnd();
+                return TASK_UNMAPPED_BASE + rnd;
 }
 
 /*
@@ -112,13 +111,18 @@ static unsigned long mmap_legacy_base(void)
  */
 void arch_pick_mmap_layout(struct mm_struct *mm)
 {
-        mm->mmap_legacy_base = mmap_legacy_base();
-        mm->mmap_base = mmap_base();
+        unsigned long random_factor = 0UL;
+
+        if (current->flags & PF_RANDOMIZE)
+                random_factor = mmap_rnd();
+
+        mm->mmap_legacy_base = mmap_legacy_base(random_factor);
 
         if (mmap_is_legacy()) {
                 mm->mmap_base = mm->mmap_legacy_base;
                 mm->get_unmapped_area = arch_get_unmapped_area;
         } else {
+                mm->mmap_base = mmap_base(random_factor);
                 mm->get_unmapped_area = arch_get_unmapped_area_topdown;
         }
 }