kasan: don't tag stacks allocated with pagealloc

Patch series "kasan: support stack instrumentation for tag-based mode", v2.

This patch (of 5):

Prepare Software Tag-Based KASAN for stack tagging support.

With Tag-Based KASAN when kernel stacks are allocated via pagealloc (which
happens when CONFIG_VMAP_STACK is not enabled), they get tagged.  KASAN
instrumentation doesn't expect the sp register to be tagged, and this
leads to false-positive reports.

Fix by resetting the tag of kernel stack pointers after allocation.
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Marco Elver <elver@google.com>
Cc: Walter Wu <walter-zh.wu@mediatek.com>
Cc: Elena Petrova <lenaptr@google.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Link: http://lkml.kernel.org/r/cover.1596199677.git.andreyknvl@google.com
Link: http://lkml.kernel.org/r/cover.1596544734.git.andreyknvl@google.com
Link: http://lkml.kernel.org/r/12d8c678869268dd0884b01271ab592f30792abf.1596544734.git.andreyknvl@google.com
Link: http://lkml.kernel.org/r/01c678b877755bcf29009176592402cdf6f2cb15.1596199677.git.andreyknvl@google.com
Link: https://bugzilla.kernel.org/show_bug.cgi?id=203497Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/fork.c

@@ -261,7 +261,7 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
 					     THREAD_SIZE_ORDER);
 
 	if (likely(page)) {
-		tsk->stack = page_address(page);
+		tsk->stack = kasan_reset_tag(page_address(page));
 		return tsk->stack;
 	}
 	return NULL;
@@ -302,6 +302,7 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk,
 {
 	unsigned long *stack;
 	stack = kmem_cache_alloc_node(thread_stack_cache, THREADINFO_GFP, node);
+	stack = kasan_reset_tag(stack);
 	tsk->stack = stack;
 	return stack;
 }