From 48ac3c18cc62d4a23d5dc5c59f8720589d0de14b Mon Sep 17 00:00:00 2001
From: Mark Rutland <mark.rutland@arm.com>
Date: Fri, 14 Jul 2017 12:23:09 +0100
Subject: [PATCH] fork: allow arch-override of VMAP stack alignment

In some cases, an architecture might wish its stacks to be aligned to a
boundary larger than THREAD_SIZE. For example, using an alignment of
double THREAD_SIZE can allow for stack overflows smaller than
THREAD_SIZE to be detected by checking a single bit of the stack
pointer.

This patch allows architectures to override the alignment of VMAP'd
stacks, by defining THREAD_ALIGN. Where not defined, this defaults to
THREAD_SIZE, as is the case today.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Will Deacon <will.deacon@arm.com>
Tested-by: Laura Abbott <labbott@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: linux-kernel@vger.kernel.org
---
 include/linux/thread_info.h | 4 ++++
 kernel/fork.c               | 3 ++-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
index 250a27614328..905d769d8ddc 100644
--- a/include/linux/thread_info.h
+++ b/include/linux/thread_info.h
@@ -38,6 +38,10 @@ enum {
 
 #ifdef __KERNEL__
 
+#ifndef THREAD_ALIGN
+#define THREAD_ALIGN	THREAD_SIZE
+#endif
+
 #ifdef CONFIG_DEBUG_STACK_USAGE
 # define THREADINFO_GFP		(GFP_KERNEL_ACCOUNT | __GFP_NOTRACK | \
 				 __GFP_ZERO)
diff --git a/kernel/fork.c b/kernel/fork.c
index 17921b0390b4..f12882a2323b 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -88,6 +88,7 @@
 #include <linux/sysctl.h>
 #include <linux/kcov.h>
 #include <linux/livepatch.h>
+#include <linux/thread_info.h>
 
 #include <asm/pgtable.h>
 #include <asm/pgalloc.h>
@@ -217,7 +218,7 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
 		return s->addr;
 	}
 
-	stack = __vmalloc_node_range(THREAD_SIZE, THREAD_SIZE,
+	stack = __vmalloc_node_range(THREAD_SIZE, THREAD_ALIGN,
 				     VMALLOC_START, VMALLOC_END,
 				     THREADINFO_GFP,
 				     PAGE_KERNEL,