[RFC PATCH 03/15] fork: Move vm_stack to the beginning of the stack

[Mostafa Saleh <smostafa@google.com>]

From: David Stevens <stevensd@google.com>

The vm_stack struct used to free stacks via an RCU callback is stored
directly in the stack being freed. Make sure it's stored at the
beginning of the stack regardless of stack growth direction, to avoid
faults on partially allocated stacks.

Signed-off-by: David Stevens <stevensd@google.com>
Signed-off-by: Mostafa Saleh <smostafa@google.com>
---
 kernel/fork.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index ec4431bf309a..473878d7f62f 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -285,7 +285,12 @@ static void thread_stack_free_rcu(struct rcu_head *rh)
 
 static void thread_stack_delayed_free(struct task_struct *tsk)
 {
-	struct vm_stack *vm_stack = tsk->stack;
+	struct vm_stack *vm_stack;
+
+	if (IS_ENABLED(CONFIG_STACK_GROWSUP))
+		vm_stack = tsk->stack;
+	else
+		vm_stack = tsk->stack + THREAD_SIZE - sizeof(*vm_stack);
 
 	vm_stack->stack_vm_area = tsk->stack_vm_area;
 	call_rcu(&vm_stack->rcu, thread_stack_free_rcu);
-- 
2.56.0.rc1.315.gc6ed9934b7-goog