[PATCH 2/2] x86/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/x86/mm/mmap.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
index db3165714521..98a2875c37e3 100644
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c
@@ -31,6 +31,7 @@
 #include <linux/sched/signal.h>
 #include <linux/sched/mm.h>
 #include <linux/compat.h>
+#include <linux/sizes.h>
 #include <asm/elf.h>
 
 #include "physaddr.h"
@@ -97,6 +98,9 @@ static unsigned long mmap_base(unsigned long rnd, unsigned long task_size,
 	unsigned long pad = stack_maxrandom_size(task_size) + stack_guard_gap;
 	unsigned long gap_min, gap_max;
 
+	/* Provide space for brk randomization */
+	pad += SZ_32M;
+
 	/* Values close to RLIM_INFINITY can overflow. */
 	if (gap + pad > gap)
 		gap += pad;
-- 
2.15.3.AMZN