[PATCHv6 1/5] x86/mm: introduce arch_rnd() to compute 32/64 mmap rnd

Dmitry Safonov
2017-03-06T14:21:23Z

To fix 32-bit mmap() syscall returning pointer higher than 4Gb in
64-bit binaries, two mmap bases will be used: one for mapping with
32-bit syscalls and another for 64-bit syscall.
To correctly place those two bases, introduce arch_rnd() function,
which will return the random factor independently of mmap_is_ia32().

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
---
arch/x86/mm/mmap.c | 26 ++++++++++++++------------
1 file changed, 14 insertions(+), 12 deletions(-)

diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
index 7940166c799b..f31ed7097d0b 100644
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c
@@ -55,6 +55,14 @@ static unsigned long stack_maxrandom_size(void)
#define MIN_GAP (128*1024*1024UL + stack_maxrandom_size())
#define MAX_GAP (TASK_SIZE/6*5)

+#ifdef CONFIG_COMPAT
+# define mmap32_rnd_bits mmap_rnd_compat_bits
+# define mmap64_rnd_bits mmap_rnd_bits
+#else
+# define mmap32_rnd_bits mmap_rnd_bits
+# define mmap64_rnd_bits mmap_rnd_bits
+#endif
+
static int mmap_is_legacy(void)
{
if (current->personality & ADDR_COMPAT_LAYOUT)
@@ -66,20 +74,14 @@ static int mmap_is_legacy(void)
return sysctl_legacy_va_layout;
}

-unsigned long arch_mmap_rnd(void)
+static unsigned long arch_rnd(unsigned int rndbits)
{
- unsigned long rnd;
-
- if (mmap_is_ia32())
-#ifdef CONFIG_COMPAT
- rnd = get_random_long() & ((1UL << mmap_rnd_compat_bits) - 1);
-#else
- rnd = get_random_long() & ((1UL << mmap_rnd_bits) - 1);
-#endif
- else
- rnd = get_random_long() & ((1UL << mmap_rnd_bits) - 1);
+ return (get_random_long() & ((1UL << rndbits) - 1)) << PAGE_SHIFT;
+}

- return rnd << PAGE_SHIFT;
+unsigned long arch_mmap_rnd(void)
+{
+ return arch_rnd(mmap_is_ia32() ? mmap32_rnd_bits : mmap64_rnd_bits);
}

static unsigned long mmap_base(unsigned long rnd)
--
2.11.1