[PATCH v4 02/10] x86: standardize mmap_rnd() usage

[Kees Cook]

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 <keesc...@chromium.org>
---
 arch/x86/mm/mmap.c | 36 ++++++++++++++++++++----------------
 1 file 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;
        }
 }
-- 
1.9.1

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