[RFC PATCH 02/15] fork: Don't assume fully populated stack during reuse

[Mostafa Saleh <smostafa@google.com>]

From: David Stevens <stevensd@google.com>

In preparation for partially populated kernel stacks, don't assume
that vm_area->nr_pages matches THREAD_SIZE when clearing a stack
for reuse.
Note that the VA range is still THREAD_SIZE in that case but it
might not be fully allocated.

Signed-off-by: David Stevens <stevensd@google.com>
[Resolve conflict and updated commit msg]
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 2065693e762c..ec4431bf309a 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -335,11 +335,16 @@ 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 offset = 0;
+
 		if (memcg_charge_kernel_stack(vm_area)) {
 			vfree(vm_area->addr);
 			return -ENOMEM;
 		}
 
+		if (!IS_ENABLED(CONFIG_STACK_GROWSUP))
+			offset = THREAD_SIZE - vm_area->nr_pages * PAGE_SIZE;
+
 		/* Reset stack metadata. */
 		if (!kasan_hw_tags_enabled())
 			kasan_unpoison_range(vm_area->addr, THREAD_SIZE);
@@ -347,7 +352,7 @@ 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. */
-		clear_pages(vm_area->addr, vm_area->nr_pages);
+		clear_pages(vm_area->addr + offset, vm_area->nr_pages);
 
 		tsk->stack_vm_area = vm_area;
 		tsk->stack = stack;
-- 
2.56.0.rc1.315.gc6ed9934b7-goog