module: Introduce module unload taint tracking

Currently, only the initial module that tainted the kernel is
recorded e.g. when an out-of-tree module is loaded.

The purpose of this patch is to allow the kernel to maintain a record of
each unloaded module that taints the kernel. So, in addition to
displaying a list of linked modules (see print_modules()) e.g. in the
event of a detected bad page, unloaded modules that carried a taint/or
taints are displayed too. A tainted module unload count is maintained.

The number of tracked modules is not fixed. This feature is disabled by
default.

Signed-off-by: Aaron Tomlin <[email protected]>
Signed-off-by: Luis Chamberlain <[email protected]>

init/Kconfig

@@ -2118,6 +2118,17 @@ config MODULE_FORCE_UNLOAD
 	  rmmod).  This is mainly for kernel developers and desperate users.
 	  If unsure, say N.
 
+config MODULE_UNLOAD_TAINT_TRACKING
+	bool "Tainted module unload tracking"
+	depends on MODULE_UNLOAD
+	default n
+	help
+	  This option allows you to maintain a record of each unloaded
+	  module that tainted the kernel. In addition to displaying a
+	  list of linked (or loaded) modules e.g. on detection of a bad
+	  page (see bad_page()), the aforementioned details are also
+	  shown. If unsure, say N.
+
 config MODVERSIONS
 	bool "Module versioning support"
 	help

kernel/module/Makefile

@@ -18,3 +18,4 @@ obj-$(CONFIG_PROC_FS) += procfs.o
 obj-$(CONFIG_SYSFS) += sysfs.o
 obj-$(CONFIG_KGDB_KDB) += kdb.o
 obj-$(CONFIG_MODVERSIONS) += version.o
+obj-$(CONFIG_MODULE_UNLOAD_TAINT_TRACKING) += tracking.o

kernel/module/internal.h

@@ -145,6 +145,27 @@ static inline bool set_livepatch_module(struct module *mod)
 #endif
 }
 
+#ifdef CONFIG_MODULE_UNLOAD_TAINT_TRACKING
+struct mod_unload_taint {
+	struct list_head list;
+	char name[MODULE_NAME_LEN];
+	unsigned long taints;
+	u64 count;
+};
+
+int try_add_tainted_module(struct module *mod);
+void print_unloaded_tainted_modules(void);
+#else /* !CONFIG_MODULE_UNLOAD_TAINT_TRACKING */
+static inline int try_add_tainted_module(struct module *mod)
+{
+	return 0;
+}
+
+static inline void print_unloaded_tainted_modules(void)
+{
+}
+#endif /* CONFIG_MODULE_UNLOAD_TAINT_TRACKING */
+
 #ifdef CONFIG_MODULE_DECOMPRESS
 int module_decompress(struct load_info *info, const void *buf, size_t size);
 void module_decompress_cleanup(struct load_info *info);

kernel/module/main.c

@@ -1190,6 +1190,9 @@ static void free_module(struct module *mod)
 	module_bug_cleanup(mod);
 	/* Wait for RCU-sched synchronizing before releasing mod->list and buglist. */
 	synchronize_rcu();
+	if (try_add_tainted_module(mod))
+		pr_err("%s: adding tainted module to the unloaded tainted modules list failed.\n",
+		       mod->name);
 	mutex_unlock(&module_mutex);
 
 	/* Clean up CFI for the module. */
@@ -3125,6 +3128,8 @@ void print_modules(void)
 			continue;
 		pr_cont(" %s%s", mod->name, module_flags(mod, buf));
 	}
+
+	print_unloaded_tainted_modules();
 	preempt_enable();
 	if (last_unloaded_module[0])
 		pr_cont(" [last unloaded: %s]", last_unloaded_module);

kernel/module/tracking.c

@@ -0,0 +1,61 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Module taint unload tracking support
+ *
+ * Copyright (C) 2022 Aaron Tomlin
+ */
+
+#include <linux/module.h>
+#include <linux/string.h>
+#include <linux/printk.h>
+#include <linux/slab.h>
+#include <linux/list.h>
+#include <linux/rculist.h>
+#include "internal.h"
+
+static LIST_HEAD(unloaded_tainted_modules);
+
+int try_add_tainted_module(struct module *mod)
+{
+	struct mod_unload_taint *mod_taint;
+
+	module_assert_mutex_or_preempt();
+
+	list_for_each_entry_rcu(mod_taint, &unloaded_tainted_modules, list,
+				lockdep_is_held(&module_mutex)) {
+		if (!strcmp(mod_taint->name, mod->name) &&
+		    mod_taint->taints & mod->taints) {
+			mod_taint->count++;
+			goto out;
+		}
+	}
+
+	mod_taint = kmalloc(sizeof(*mod_taint), GFP_KERNEL);
+	if (unlikely(!mod_taint))
+		return -ENOMEM;
+	strscpy(mod_taint->name, mod->name, MODULE_NAME_LEN);
+	mod_taint->taints = mod->taints;
+	list_add_rcu(&mod_taint->list, &unloaded_tainted_modules);
+	mod_taint->count = 1;
+out:
+	return 0;
+}
+
+void print_unloaded_tainted_modules(void)
+{
+	struct mod_unload_taint *mod_taint;
+	char buf[MODULE_FLAGS_BUF_SIZE];
+
+	if (!list_empty(&unloaded_tainted_modules)) {
+		printk(KERN_DEFAULT "Unloaded tainted modules:");
+		list_for_each_entry_rcu(mod_taint, &unloaded_tainted_modules,
+					list) {
+			size_t l;
+
+			l = module_flags_taint(mod_taint->taints, buf);
+			buf[l++] = '\0';
+			pr_cont(" %s(%s):%llu", mod_taint->name, buf,
+				mod_taint->count);
+		}
+	}
+}