From d2c5b3b25b188f27175011186c88b1002df25ac4 Mon Sep 17 00:00:00 2001
From: David Carlier <devnexen@gmail.com>
Date: Sat, 15 Nov 2025 15:14:25 +0000
Subject: [PATCH] Fix GH-20483: ASAN stack overflow with small fiber.stack_size
 INI value.

close GH-20495
---
 NEWS                           |  4 ++++
 Zend/tests/fibers/gh20483.phpt | 16 ++++++++++++++++
 Zend/zend_fibers.c             |  7 ++++++-
 3 files changed, 26 insertions(+), 1 deletion(-)
 create mode 100644 Zend/tests/fibers/gh20483.phpt

diff --git a/NEWS b/NEWS
index c9ff9ac461d..5ddc2633d19 100644
--- a/NEWS
+++ b/NEWS
@@ -14,6 +14,10 @@ PHP                                                                        NEWS
 - DOM:
   . Fix missing NUL byte check on C14NFile(). (ndossche)
 
+- Fibers:
+  . Fixed bug GH-20483 (ASAN stack overflow with fiber.stack_size INI
+    small value). (David Carlier)
+
 - Opcache:
   . Fixed bug GH-20329 (opcache.file_cache broken with full interned string
     buffer). (Arnaud)
diff --git a/Zend/tests/fibers/gh20483.phpt b/Zend/tests/fibers/gh20483.phpt
new file mode 100644
index 00000000000..e06cf87258e
--- /dev/null
+++ b/Zend/tests/fibers/gh20483.phpt
@@ -0,0 +1,16 @@
+--TEST--
+GH-20483 (ASAN stack overflow with small fiber.stack_size INI value)
+--INI--
+fiber.stack_size=1024
+--FILE--
+<?php
+$callback = function () {};
+$fiber = new Fiber($callback);
+try {
+    $fiber->start();
+} catch (Exception $e) {
+    echo $e->getMessage() . "\n";
+}
+?>
+--EXPECTF--
+Fiber stack size is too small, it needs to be at least %d bytes
diff --git a/Zend/zend_fibers.c b/Zend/zend_fibers.c
index 6b6c1eaae1a..96f6e99e714 100644
--- a/Zend/zend_fibers.c
+++ b/Zend/zend_fibers.c
@@ -206,7 +206,12 @@ static zend_fiber_stack *zend_fiber_stack_allocate(size_t size)
 {
 	void *pointer;
 	const size_t page_size = zend_fiber_get_page_size();
-	const size_t minimum_stack_size = page_size + ZEND_FIBER_GUARD_PAGES * page_size;
+	const size_t minimum_stack_size = page_size + ZEND_FIBER_GUARD_PAGES * page_size
+#ifdef __SANITIZE_ADDRESS__
+	// necessary correction due to ASAN redzones
+	* 6
+#endif
+	;
 
 	if (size < minimum_stack_size) {
 		zend_throw_exception_ex(NULL, 0, "Fiber stack size is too small, it needs to be at least %zu bytes", minimum_stack_size);