[PATCH 1/2] arm64/mmap: handle worst-case heap randomization in mmap_base

[Ali Saidi]

Increase mmap_base by the worst-case brk randomization so that
the stack and heap remain apart.

In Linux 4.13 a change was committed that special cased the kernel ELF
loader when the loader is invoked directly (eab09532d400; binfmt_elf: use
ELF_ET_DYN_BASE only for PIE). Generally, the loader isnât invoked
directly and this issue is limited to cases where it is, (e.g to set a
non-inheritable LD_LIBRARY_PATH, testing new versions of the loader). In
those rare cases, the loader doesn't take into account the amount of brk
randomization that will be applied by arch_randomize_brk(). This can
lead to the stack and heap being arbitrarily close to each other.

Signed-off-by: Ali Saidi <alisaidi@xxxxxxxxxx>

---
 arch/arm64/mm/mmap.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/arm64/mm/mmap.c b/arch/arm64/mm/mmap.c
index 842c8a5fcd53..0778f7ba8306 100644
--- a/arch/arm64/mm/mmap.c
+++ b/arch/arm64/mm/mmap.c
@@ -67,6 +67,14 @@ static unsigned long mmap_base(unsigned long rnd, struct rlimit *rlim_stack)
 	unsigned long gap = rlim_stack->rlim_cur;
 	unsigned long pad = (STACK_RND_MASK << PAGE_SHIFT) + stack_guard_gap;
 
+	/* Provide space for randomization when randomize_va_space == 2 and
+	 * ld-linux.so is called directly. Values from arch_randomize_brk()
+	 */
+	if (test_thread_flag(TIF_32BIT))
+		pad += SZ_32M;
+	else
+		pad += SZ_1G;
+
 	/* Values close to RLIM_INFINITY can overflow. */
 	if (gap + pad > gap)
 		gap += pad;
-- 
2.15.3.AMZN