[PATCH v6 09/14] mips: Properly account for stack randomization and stack guard gap

[Alexandre Ghiti]

This commit takes care of stack randomization and stack guard gap when
computing mmap base address and checks if the task asked for randomization.

This fixes the problem uncovered and not fixed for arm here:
https://lkml.kernel.org/r/20170622200033.25714-1-riel@xxxxxxxxxx

Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>
Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
Acked-by: Paul Burton <paul.burton@xxxxxxxx>
Reviewed-by: Luis Chamberlain <mcgrof@xxxxxxxxxx>
---
 arch/mips/mm/mmap.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/mips/mm/mmap.c b/arch/mips/mm/mmap.c
index d79f2b432318..f5c778113384 100644
--- a/arch/mips/mm/mmap.c
+++ b/arch/mips/mm/mmap.c
@@ -21,8 +21,9 @@ unsigned long shm_align_mask = PAGE_SIZE - 1;	/* Sane caches */
 EXPORT_SYMBOL(shm_align_mask);
 
 /* gap between mmap and stack */
-#define MIN_GAP (128*1024*1024UL)
-#define MAX_GAP ((TASK_SIZE)/6*5)
+#define MIN_GAP		(128*1024*1024UL)
+#define MAX_GAP		((TASK_SIZE)/6*5)
+#define STACK_RND_MASK	(0x7ff >> (PAGE_SHIFT - 12))
 
 static int mmap_is_legacy(struct rlimit *rlim_stack)
 {
@@ -38,6 +39,15 @@ static int mmap_is_legacy(struct rlimit *rlim_stack)
 static unsigned long mmap_base(unsigned long rnd, struct rlimit *rlim_stack)
 {
 	unsigned long gap = rlim_stack->rlim_cur;
+	unsigned long pad = stack_guard_gap;
+
+	/* Account for stack randomization if necessary */
+	if (current->flags & PF_RANDOMIZE)
+		pad += (STACK_RND_MASK << PAGE_SHIFT);
+
+	/* Values close to RLIM_INFINITY can overflow. */
+	if (gap + pad > gap)
+		gap += pad;
 
 	if (gap < MIN_GAP)
 		gap = MIN_GAP;
-- 
2.20.1