[PATCH v2 02/13] fork: Don't assume fully populated stack during reuse

[David Stevens <stevensd@google.com>]

In preparation for dynamic kernel stacks, don't assume that
vm_area->nr_pages matches THREAD_SIZE when clearing a stack for reuse.

Signed-off-by: David Stevens <stevensd@google.com>
---
 kernel/fork.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 8961b895bf05..50772c0cc5da 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -332,6 +332,8 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node)
 
 	vm_area = alloc_thread_stack_node_from_cache(tsk, node);
 	if (vm_area) {
+		unsigned long memset_offset = 0;
+
 		if (memcg_charge_kernel_stack(vm_area)) {
 			vfree(vm_area->addr);
 			return -ENOMEM;
@@ -343,7 +345,9 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node)
 		stack = kasan_reset_tag(vm_area->addr);
 
 		/* Clear stale pointers from reused stack. */
-		memset(stack, 0, THREAD_SIZE);
+		if (!IS_ENABLED(CONFIG_STACK_GROWSUP))
+			memset_offset = THREAD_SIZE - vm_area->nr_pages * PAGE_SIZE;
+		memset(stack + memset_offset, 0, vm_area->nr_pages * PAGE_SIZE);
 
 		tsk->stack_vm_area = vm_area;
 		tsk->stack = stack;
-- 
2.54.0.rc2.544.gc7ae2d5bb8-goog