Refresh zend_mm shadow key on fork

The shadow key is refreshed when resetting the memory manager between two
requests. But in forking SAPIs the first request of a child process inherits the
shadow key of the parent. As a result, a leak of the shadow key during the first
request of one process gives away the shadow key used during the first request
of other processes. This makes the key refresh mechanism less useful.

Here I ensure that we refresh the shadow key after a fork. We can not reset the
manager as there may be active allocations. Instead, we have to recompute shadow
pointers with the new key.

Closes GH-16765

Author: arnaud-lb

UPGRADING.INTERNALS

@@ -73,6 +73,8 @@ PHP 8.5 INTERNALS UPGRADE NOTES
   . EG(fake_scope) now is a _const_ zend_class_entry*.
   . zend_begin_record_errors() or EG(record_errors)=true cause errors to be
     delayed. Before, errors would be recorded but not delayed.
+  . zend_mm_refresh_key_child() must be called on any zend_mm_heap inherited
+    from the parent process after a fork().
 
 ========================
 2. Build system changes
@@ -148,3 +150,7 @@ PHP 8.5 INTERNALS UPGRADE NOTES
 ========================
 5. SAPI changes
 ========================
+
+- SAPIs must now call php_child_init() after a fork. If php-src code was
+  executed in other threads than the one initiating the fork,
+  refresh_memory_manager() must be called in every such thread.

Zend/zend_alloc.c

@@ -317,7 +317,9 @@ struct _zend_mm_heap {
 		} debug;
 	};
 #endif
+#if ZEND_DEBUG
 	pid_t pid;
+#endif
 	zend_random_bytes_insecure_state rand_state;
 };
 
@@ -1310,15 +1312,20 @@ static zend_always_inline zend_mm_free_slot* zend_mm_encode_free_slot(const zend
 #endif
 }
 
-static zend_always_inline zend_mm_free_slot* zend_mm_decode_free_slot(zend_mm_heap *heap, zend_mm_free_slot *slot)
+static zend_always_inline zend_mm_free_slot* zend_mm_decode_free_slot_key(uintptr_t shadow_key, zend_mm_free_slot *slot)
 {
 #ifdef WORDS_BIGENDIAN
-	return (zend_mm_free_slot*)((uintptr_t)slot ^ heap->shadow_key);
+	return (zend_mm_free_slot*)((uintptr_t)slot ^ shadow_key);
 #else
-	return (zend_mm_free_slot*)(BSWAPPTR((uintptr_t)slot ^ heap->shadow_key));
+	return (zend_mm_free_slot*)(BSWAPPTR((uintptr_t)slot ^ shadow_key));
 #endif
 }
 
+static zend_always_inline zend_mm_free_slot* zend_mm_decode_free_slot(zend_mm_heap *heap, zend_mm_free_slot *slot)
+{
+	return zend_mm_decode_free_slot_key(heap->shadow_key, slot);
+}
+
 static zend_always_inline void zend_mm_set_next_free_slot(zend_mm_heap *heap, uint32_t bin_num, zend_mm_free_slot *slot, zend_mm_free_slot *next)
 {
 	ZEND_ASSERT(bin_data_size[bin_num] >= ZEND_MM_MIN_USEABLE_BIN_SIZE);
@@ -2027,6 +2034,34 @@ static void zend_mm_init_key(zend_mm_heap *heap)
 	zend_mm_refresh_key(heap);
 }
 
+ZEND_API void zend_mm_refresh_key_child(zend_mm_heap *heap)
+{
+	uintptr_t old_key = heap->shadow_key;
+
+	zend_mm_init_key(heap);
+
+	/* Update shadow pointers with new key */
+	for (int i = 0; i < ZEND_MM_BINS; i++) {
+		zend_mm_free_slot *slot = heap->free_slot[i];
+		if (!slot) {
+			continue;
+		}
+		zend_mm_free_slot *next;
+		while ((next = slot->next_free_slot)) {
+			zend_mm_free_slot *shadow = ZEND_MM_FREE_SLOT_PTR_SHADOW(slot, i);
+			if (UNEXPECTED(next != zend_mm_decode_free_slot_key(old_key, shadow))) {
+				zend_mm_panic("zend_mm_heap corrupted");
+			}
+			zend_mm_set_next_free_slot(heap, i, slot, next);
+			slot = next;
+		}
+	}
+
+#if ZEND_DEBUG
+	heap->pid = getpid();
+#endif
+}
+
 static zend_mm_heap *zend_mm_init(void)
 {
 	zend_mm_chunk *chunk = (zend_mm_chunk*)zend_mm_chunk_alloc_int(ZEND_MM_CHUNK_SIZE, ZEND_MM_CHUNK_SIZE);
@@ -2075,7 +2110,9 @@ static zend_mm_heap *zend_mm_init(void)
 	heap->storage = NULL;
 #endif
 	heap->huge_list = NULL;
+#if ZEND_DEBUG
 	heap->pid = getpid();
+#endif
 	return heap;
 }
 
@@ -2535,13 +2572,12 @@ ZEND_API void zend_mm_shutdown(zend_mm_heap *heap, bool full, bool silent)
 		p->free_map[0] = (1L << ZEND_MM_FIRST_PAGE) - 1;
 		p->map[0] = ZEND_MM_LRUN(ZEND_MM_FIRST_PAGE);
 
-		pid_t pid = getpid();
-		if (heap->pid != pid) {
-			zend_mm_init_key(heap);
-			heap->pid = pid;
-		} else {
-			zend_mm_refresh_key(heap);
-		}
+#if ZEND_DEBUG
+		ZEND_ASSERT(getpid() == heap->pid
+				&& "heap was re-used without calling zend_mm_refresh_key_child() after a fork");
+#endif
+
+		zend_mm_refresh_key(heap);
 	}
 }
 
@@ -2949,6 +2985,11 @@ ZEND_API void shutdown_memory_manager(bool silent, bool full_shutdown)
 	zend_mm_shutdown(AG(mm_heap), full_shutdown, silent);
 }
 
+ZEND_API void refresh_memory_manager(void)
+{
+	zend_mm_refresh_key_child(AG(mm_heap));
+}
+
 static ZEND_COLD ZEND_NORETURN void zend_out_of_memory(void)
 {
 	fprintf(stderr, "Out of memory\n");
@@ -3506,7 +3547,9 @@ ZEND_API zend_mm_heap *zend_mm_startup_ex(const zend_mm_handlers *handlers, void
 		memcpy(storage->data, data, data_size);
 	}
 	heap->storage = storage;
+#if ZEND_DEBUG
 	heap->pid = getpid();
+#endif
 	return heap;
 #else
 	return NULL;

Zend/zend_alloc.h

@@ -220,6 +220,7 @@ ZEND_API bool zend_alloc_in_memory_limit_error_reporting(void);
 
 ZEND_API void start_memory_manager(void);
 ZEND_API void shutdown_memory_manager(bool silent, bool full_shutdown);
+ZEND_API void refresh_memory_manager(void);
 ZEND_API bool is_zend_mm(void);
 ZEND_API bool is_zend_ptr(const void *ptr);
 
@@ -316,6 +317,8 @@ struct _zend_mm_storage {
 ZEND_API zend_mm_storage *zend_mm_get_storage(zend_mm_heap *heap);
 ZEND_API zend_mm_heap *zend_mm_startup_ex(const zend_mm_handlers *handlers, void *data, size_t data_size);
 
+ZEND_API void zend_mm_refresh_key_child(zend_mm_heap *heap);
+
 /*
 
 // The following example shows how to use zend_mm_heap API with custom storage

ext/pcntl/pcntl.c

@@ -32,6 +32,7 @@
 #include "php_signal.h"
 #include "php_ticks.h"
 #include "zend_fibers.h"
+#include "main/php_main.h"
 
 #if defined(HAVE_GETPRIORITY) || defined(HAVE_SETPRIORITY) || defined(HAVE_WAIT3)
 #include <sys/wait.h>
@@ -297,7 +298,7 @@ PHP_FUNCTION(pcntl_fork)
 
 		}
 	} else if (id == 0) {
-		zend_max_execution_timer_init();
+		php_child_init();
 	}
 
 	RETURN_LONG((zend_long) id);
@@ -1560,7 +1561,7 @@ PHP_FUNCTION(pcntl_rfork)
 			php_error_docref(NULL, E_WARNING, "Error %d", errno);
 		}
 	} else if (pid == 0) {
-		zend_max_execution_timer_init();
+		php_child_init();
 	}
 
 	RETURN_LONG((zend_long) pid);
@@ -1605,7 +1606,7 @@ PHP_FUNCTION(pcntl_forkx)
 			php_error_docref(NULL, E_WARNING, "Error %d", errno);
 		}
 	} else if (pid == 0) {
-		zend_max_execution_timer_init();
+		php_child_init();
 	}
 
 	RETURN_LONG((zend_long) pid);

main/main.c

@@ -1816,6 +1816,12 @@ static void sigchld_handler(int apar)
 /* }}} */
 #endif
 
+PHPAPI void php_child_init(void)
+{
+	refresh_memory_manager();
+	zend_max_execution_timer_init();
+}
+
 /* {{{ php_request_startup */
 zend_result php_request_startup(void)
 {

main/php_main.h

@@ -49,6 +49,7 @@ ZEND_ATTRIBUTE_CONST PHPAPI const char *php_build_provider(void);
 PHPAPI char *php_get_version(sapi_module_struct *sapi_module);
 PHPAPI void php_print_version(sapi_module_struct *sapi_module);
 
+PHPAPI void php_child_init(void);
 PHPAPI zend_result php_request_startup(void);
 PHPAPI void php_request_shutdown(void *dummy);
 PHPAPI zend_result php_module_startup(sapi_module_struct *sf, zend_module_entry *additional_module);

sapi/apache2handler/sapi_apache2.c

@@ -752,6 +752,7 @@ zend_first_try {
 static void php_apache_child_init(apr_pool_t *pchild, server_rec *s)
 {
 	apr_pool_cleanup_register(pchild, NULL, php_apache_child_shutdown, apr_pool_cleanup_null);
+	php_child_init();
 }
 
 #ifdef ZEND_SIGNALS

sapi/cgi/cgi_main.c

@@ -2041,6 +2041,8 @@ consult the installation file that came with this distribution, or visit \n\
 						 */
 						parent = 0;
 
+						php_child_init();
+
 						/* don't catch our signals */
 						sigaction(SIGTERM, &old_term, 0);
 						sigaction(SIGQUIT, &old_quit, 0);

sapi/cli/php_cli_server.c

@@ -2530,6 +2530,7 @@ static void php_cli_server_startup_workers(void) {
 #if defined(HAVE_PRCTL) || defined(HAVE_PROCCTL)
 				php_cli_server_worker_install_pdeathsig();
 #endif
+				php_child_init();
 				return;
 			} else {
 				php_cli_server_workers[php_cli_server_worker] = pid;

sapi/fpm/fpm/fpm_php.c

@@ -253,6 +253,9 @@ int fpm_php_init_child(struct fpm_worker_pool_s *wp) /* {{{ */
 		limit_extensions = wp->limit_extensions;
 		wp->limit_extensions = NULL;
 	}
+
+	php_child_init();
+
 	return 0;
 }
 /* }}} */