[PATCH AUTOSEL 4.19 32/33] arm: properly account for stack randomization and stack guard gap

[Sasha Levin]

From: Alexandre Ghiti <alex@xxxxxxxx>

[ Upstream commit af0f4297286f13a75edf93677b1fb2fc16c412a7 ]

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

Link: http://lkml.kernel.org/r/20190730055113.23635-7-alex@xxxxxxxx
Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>
Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
Reviewed-by: Luis Chamberlain <mcgrof@xxxxxxxxxx>
Cc: Albert Ou <aou@xxxxxxxxxxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: James Hogan <jhogan@xxxxxxxxxx>
Cc: Palmer Dabbelt <palmer@xxxxxxxxxx>
Cc: Paul Burton <paul.burton@xxxxxxxx>
Cc: Ralf Baechle <ralf@xxxxxxxxxxxxxx>
Cc: Russell King <linux@xxxxxxxxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 arch/arm/mm/mmap.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/arm/mm/mmap.c b/arch/arm/mm/mmap.c
index f866870db749c..bff3d00bda5be 100644
--- a/arch/arm/mm/mmap.c
+++ b/arch/arm/mm/mmap.c
@@ -18,8 +18,9 @@
 	 (((pgoff)<<PAGE_SHIFT) & (SHMLBA-1)))
 
 /* 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)
 {
@@ -35,6 +36,15 @@ 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