[PATCH v3] livepatch: add (un)patch callbacks

[Joe Lawrence]

Provide livepatch modules a klp_object (un)patching notification
mechanism.  Pre and post-(un)patch callbacks allow livepatch modules to
setup or synchronize changes that would be difficult to support in only
patched-or-unpatched code contexts.

Callbacks can be registered for target module or vmlinux klp_objects,
but each implementation is klp_object specific.

  - Pre-(un)patch callbacks run before any (un)patching action takes
    place.

  - Post-(un)patch callbacks run once an object has been (un)patched and
    the klp_patch fully transitioned to its target state.

Example use cases include modification of global data and registration
of newly available services/handlers.

See Documentation/livepatch/callback.txt for details.

Signed-off-by: Joe Lawrence <joe.lawrence@xxxxxxxxxx>
---
 Documentation/livepatch/callbacks.txt        |  87 ++++++++++++
 include/linux/livepatch.h                    |  81 ++++++++++++
 kernel/livepatch/core.c                      |  37 ++++--
 kernel/livepatch/patch.c                     |   5 +-
 kernel/livepatch/transition.c                |  21 ++-
 samples/livepatch/Makefile                   |   2 +
 samples/livepatch/livepatch-callbacks-demo.c | 190 +++++++++++++++++++++++++++
 samples/livepatch/livepatch-callbacks-mod.c  |  53 ++++++++
 8 files changed, 462 insertions(+), 14 deletions(-)
 create mode 100644 Documentation/livepatch/callbacks.txt
 create mode 100644 samples/livepatch/livepatch-callbacks-demo.c
 create mode 100644 samples/livepatch/livepatch-callbacks-mod.c

diff --git a/Documentation/livepatch/callbacks.txt b/Documentation/livepatch/callbacks.txt
new file mode 100644
index 000000000000..e18f2678f3bb
--- /dev/null
+++ b/Documentation/livepatch/callbacks.txt
@@ -0,0 +1,87 @@
+(Un)patching Callbacks
+======================
+
+Livepatch (un)patch-callbacks provide a mechanism for livepatch modules
+to execute callback functions when a kernel object is (un)patched.  They
+can be considered a "power feature" that extends livepatching abilities
+to include:
+
+  - Safe updates to global data
+
+  - "Patches" to init and probe functions
+
+  - Patching otherwise unpatchable code (i.e. assembly)
+
+In most cases, (un)patch hooks will need to be used in conjunction with
+memory barriers and kernel synchronization primitives, like
+mutexes/spinlocks, or even stop_machine(), to avoid concurrency issues.
+
+Callbacks differ from existing kernel facilities:
+
+  - Module init/exit code doesn't run when disabling and re-enabling a
+    patch.
+
+  - A module notifier can't stop the to-be-patched module from loading.
+
+Callbacks are part of the klp_object structure and their implementation
+is specific to the given object.  Other livepatch objects may or may not
+be patched, irrespective of the target klp_object's current state.
+
+Callbacks can be registered for the following livepatch actions:
+
+  * Pre-patch    - before klp_object is patched
+
+  * Post-patch   - after klp_object has been patched and is active
+                   across all tasks
+
+  * Pre-unpatch  - before klp_object is unpatched, patched code is active
+
+  * Post-unpatch - after klp_object has been patched, all code has been
+		   restored and no tasks are running patched code
+
+A callbacks is only executed if its host klp_object is loaded.  For
+in-kernel vmlinux targets, this means that callbacks will always execute
+when a livepatch is enabled/disabled.
+
+For kernel module targets, callbacks will only execute if the target
+module is loaded.  When a kernel module target is (un)loaded, its
+callbacks will execute only if the livepatch module is enabled.
+
+The pre-patch callback is expected to return a status code (0 for
+success, -ERRNO on error).  An error status code will indicate to the
+livepatching core that patching of the current klp_object is not safe
+and to stop the current patching request.  If the problematic klp_object
+is already loaded (i.e. a livepatch is loaded after target code), the
+kernel's module loader will refuse to load the livepatch.  On the other
+hand, if the problematic klp_object is already in place (i.e. a target
+module is loaded after a livepatch), then the module loader will refuse
+to load the target kernel module.
+
+
+Example Use-cases
+-----------------
+
+1 - Update global data
+
+A pre-patch callback can be useful to update a global variable.  For
+example, 75ff39ccc1bd ("tcp: make challenge acks less predictable")
+changes a global sysctl, as well as patches the tcp_send_challenge_ack()
+function.
+
+In this case, if we're being super paranoid, it might make sense to
+patch the data *after* patching is complete with a post-patch callback,
+so that tcp_send_challenge_ack() could first be changed to read
+sysctl_tcp_challenge_ack_limit with READ_ONCE.
+
+
+2 - Support __init and probe function patches
+
+Although __init and probe functions are not directly livepatch-able, it
+may be possible to implement similar updates via pre/post-patch
+callbacks.
+
+48900cb6af42 ("virtio-net: drop NETIF_F_FRAGLIST") change the way that
+virtnet_probe() initialized its driver's net_device features.  A
+pre/post-patch callback could iterate over all such devices, making a
+similar change to their hw_features value.  (Client functions of the
+value may need to be updated accordingly.)
diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
index 194991ef9347..500dc9b2b361 100644
--- a/include/linux/livepatch.h
+++ b/include/linux/livepatch.h
@@ -87,10 +87,25 @@ struct klp_func {
 	bool transition;
 };
 
+struct klp_object;
+
+/**
+ * struct klp_callback - callback structure for live patching
+ * @callback:	function to be executed on callback
+ *
+ */
+struct klp_callbacks {
+	int (*pre_patch)(struct klp_object *obj);
+	void (*post_patch)(struct klp_object *obj);
+	void (*pre_unpatch)(struct klp_object *obj);
+	void (*post_unpatch)(struct klp_object *obj);
+};
+
 /**
  * struct klp_object - kernel object structure for live patching
  * @name:	module name (or NULL for vmlinux)
  * @funcs:	function entries for functions to be patched in the object
+ * @callbacks:	functions to be executed pre/post (un)patching
  * @kobj:	kobject for sysfs resources
  * @mod:	kernel module associated with the patched object
  *		(NULL for vmlinux)
@@ -100,6 +115,7 @@ struct klp_object {
 	/* external */
 	const char *name;
 	struct klp_func *funcs;
+	struct klp_callbacks callbacks;
 
 	/* internal */
 	struct kobject kobj;
@@ -138,6 +154,71 @@ struct klp_patch {
 	     func->old_name || func->new_func || func->old_sympos; \
 	     func++)
 
+/**
+ * klp_is_object_loaded() - is klp_object currently loaded?
+ * @obj:	klp_object pointer
+ *
+ * Return: true if klp_object is loaded (always true for vmlinux)
+ */
+static inline bool klp_is_object_loaded(struct klp_object *obj)
+{
+	return !obj->name || obj->mod;
+}
+
+/**
+ * klp_pre_patch_callback - execute before klp_object is patched
+ * @obj:	invoke callback for this klp_object
+ *
+ * Return: status from callback
+ *
+ * Callers should ensure obj->patched is *not* set.
+ */
+static inline int klp_pre_patch_callback(struct klp_object *obj)
+{
+	if (obj->callbacks.pre_patch)
+		return (*obj->callbacks.pre_patch)(obj);
+	return 0;
+}
+
+/**
+ * klp_post_patch_callback() - execute after klp_object is patched
+ * @obj:	invoke callback for this klp_object
+ *
+ * Callers should ensure obj->patched is set.
+ */
+static inline void klp_post_patch_callback(struct klp_object *obj)
+{
+	if (obj->callbacks.post_patch)
+		(*obj->callbacks.post_patch)(obj);
+}
+
+/**
+ * klp_pre_unpatch_callback() - execute before klp_object is unpatched
+ *                              and is active across all tasks
+ * @obj:	invoke callback for this klp_object
+ *
+ * Callers should ensure obj->patched is set.
+ */
+static inline void klp_pre_unpatch_callback(struct klp_object *obj)
+{
+	if (obj->callbacks.pre_unpatch)
+		(*obj->callbacks.pre_unpatch)(obj);
+}
+
+/**
+ * klp_post_unpatch_callback() - execute after klp_object is unpatched,
+ *                               all code has been restored and no tasks
+ *                               are running patched code
+ * @obj:	invoke callback for this klp_object
+ *
+ * Callers should ensure obj->patched is *not* set.
+ */
+static inline void klp_post_unpatch_callback(struct klp_object *obj)
+{
+	if (obj->callbacks.post_unpatch)
+		(*obj->callbacks.post_unpatch)(obj);
+}
+
 int klp_register_patch(struct klp_patch *);
 int klp_unregister_patch(struct klp_patch *);
 int klp_enable_patch(struct klp_patch *);
diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index b9628e43c78f..ddb23e18a357 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -54,11 +54,6 @@ static bool klp_is_module(struct klp_object *obj)
 	return obj->name;
 }
 
-static bool klp_is_object_loaded(struct klp_object *obj)
-{
-	return !obj->name || obj->mod;
-}
-
 /* sets obj->mod if object is not vmlinux and module is found */
 static void klp_find_object_module(struct klp_object *obj)
 {
@@ -388,13 +383,18 @@ static int __klp_enable_patch(struct klp_patch *patch)
 		if (!klp_is_object_loaded(obj))
 			continue;
 
-		ret = klp_patch_object(obj);
+		ret = klp_pre_patch_callback(obj);
 		if (ret) {
-			pr_warn("failed to enable patch '%s'\n",
-				patch->mod->name);
+			pr_warn("pre-patch callback failed for object '%s'\n",
+				klp_is_module(obj) ? obj->name : "vmlinux");
+			goto err;
+		}
 
-			klp_cancel_transition();
-			return ret;
+		ret = klp_patch_object(obj);
+		if (ret) {
+			pr_warn("failed to patch object '%s'\n",
+				klp_is_module(obj) ? obj->name : "vmlinux");
+			goto err;
 		}
 	}
 
@@ -403,6 +403,11 @@ static int __klp_enable_patch(struct klp_patch *patch)
 	patch->enabled = true;
 
 	return 0;
+err:
+	pr_warn("failed to enable patch '%s'\n", patch->mod->name);
+
+	klp_cancel_transition();
+	return ret;
 }
 
 /**
@@ -871,6 +876,13 @@ int klp_module_coming(struct module *mod)
 			pr_notice("applying patch '%s' to loading module '%s'\n",
 				  patch->mod->name, obj->mod->name);
 
+			ret = klp_pre_patch_callback(obj);
+			if (ret) {
+				pr_warn("pre-patch callback failed for object '%s'\n",
+					obj->name);
+				goto err;
+			}
+
 			ret = klp_patch_object(obj);
 			if (ret) {
 				pr_warn("failed to apply patch '%s' to module '%s' (%d)\n",
@@ -878,6 +890,8 @@ int klp_module_coming(struct module *mod)
 				goto err;
 			}
 
+			klp_post_patch_callback(obj);
+
 			break;
 		}
 	}
@@ -929,7 +943,10 @@ void klp_module_going(struct module *mod)
 			if (patch->enabled || patch == klp_transition_patch) {
 				pr_notice("reverting patch '%s' on unloading module '%s'\n",
 					  patch->mod->name, obj->mod->name);
+
+				klp_pre_unpatch_callback(obj);
 				klp_unpatch_object(obj);
+				klp_post_unpatch_callback(obj);
 			}
 
 			klp_free_object_loaded(obj);
diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c
index 52c4e907c14b..0eed0df6e6d9 100644
--- a/kernel/livepatch/patch.c
+++ b/kernel/livepatch/patch.c
@@ -257,6 +257,7 @@ int klp_patch_object(struct klp_object *obj)
 	klp_for_each_func(obj, func) {
 		ret = klp_patch_func(func);
 		if (ret) {
+			klp_pre_unpatch_callback(obj);
 			klp_unpatch_object(obj);
 			return ret;
 		}
@@ -271,6 +272,8 @@ void klp_unpatch_objects(struct klp_patch *patch)
 	struct klp_object *obj;
 
 	klp_for_each_object(patch, obj)
-		if (obj->patched)
+		if (obj->patched) {
+			klp_pre_unpatch_callback(obj);
 			klp_unpatch_object(obj);
+		}
 }
diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
index b004a1fb6032..7bf55b7f3687 100644
--- a/kernel/livepatch/transition.c
+++ b/kernel/livepatch/transition.c
@@ -109,9 +109,6 @@ static void klp_complete_transition(void)
 		}
 	}
 
-	if (klp_target_state == KLP_UNPATCHED && !immediate_func)
-		module_put(klp_transition_patch->mod);
-
 	/* Prevent klp_ftrace_handler() from seeing KLP_UNDEFINED state */
 	if (klp_target_state == KLP_PATCHED)
 		klp_synchronize_transition();
@@ -130,6 +127,24 @@ static void klp_complete_transition(void)
 	}
 
 done:
+	klp_for_each_object(klp_transition_patch, obj) {
+		if (!klp_is_object_loaded(obj))
+			continue;
+		if (klp_target_state == KLP_PATCHED)
+			klp_post_patch_callback(obj);
+		else if (klp_target_state == KLP_UNPATCHED)
+			klp_post_unpatch_callback(obj);
+	}
+
+	/*
+	 * See complementary comment in __klp_enable_patch() for why we
+	 * keep the module reference for immediate patches.
+	 */
+	if (!klp_transition_patch->immediate && !immediate_func &&
+	    klp_target_state == KLP_UNPATCHED) {
+		module_put(klp_transition_patch->mod);
+	}
+
 	klp_target_state = KLP_UNDEFINED;
 	klp_transition_patch = NULL;
 }
diff --git a/samples/livepatch/Makefile b/samples/livepatch/Makefile
index 10319d7ea0b1..f8e545af5de4 100644
--- a/samples/livepatch/Makefile
+++ b/samples/livepatch/Makefile
@@ -1 +1,3 @@
 obj-$(CONFIG_SAMPLE_LIVEPATCH) += livepatch-sample.o
+obj-$(CONFIG_SAMPLE_LIVEPATCH) += livepatch-callbacks-demo.o
+obj-$(CONFIG_SAMPLE_LIVEPATCH) += livepatch-callbacks-mod.o
diff --git a/samples/livepatch/livepatch-callbacks-demo.c b/samples/livepatch/livepatch-callbacks-demo.c
new file mode 100644
index 000000000000..a5ae7211f24a
--- /dev/null
+++ b/samples/livepatch/livepatch-callbacks-demo.c
@@ -0,0 +1,190 @@
+/*
+ * Copyright (C) 2017 Joe Lawrence <joe.lawrence@xxxxxxxxxx>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+/*
+ * livepatch-callbacks-demo.c - (un)patching callbacks livepatch demo
+ *
+ *
+ * Purpose
+ * -------
+ *
+ * Demonstration of registering livepatch (un)patching callbacks.
+ *
+ *
+ * Usage
+ * -----
+ *
+ * Step 1 - load the simple module
+ *
+ *   insmod samples/livepatch/livepatch-callbacks-mod.ko
+ *
+ *
+ * Step 2 - load the demonstration livepatch (with callbacks)
+ *
+ *   insmod samples/livepatch/livepatch-callbacks-demo.ko
+ *
+ *
+ * Step 3 - cleanup
+ *
+ *   echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled
+ *   rmmod livepatch_callbacks_demo
+ *   rmmod livepatch_callbacks_mod
+ *
+ * Watch dmesg output to see livepatch enablement, callback execution
+ * and patching operations for both vmlinux and module targets.
+ *
+ * NOTE: swap the insmod order of livepatch-callbacks-mod.ko and
+ *       livepatch-callbacks-demo.ko to observe what happens when a
+ *       target module is loaded after a livepatch with callbacks.
+ *
+ * NOTE: 'pre_patch_ret' is a module parameter that sets the pre-patch
+ *       callback return status.  Try setting up a non-zero status
+ *       such as -19 (-ENODEV):
+ *
+ *       # Load demo livepatch, vmlinux is patched
+ *       insmod samples/livepatch/livepatch-callbacks-demo.ko
+ *
+ *       # Setup next pre-patch callback to return -ENODEV
+ *       echo -19 > /sys/module/livepatch_callbacks_demo/parameters/pre_patch_ret
+ *
+ *       # Module loader refuses to load the target module
+ *       insmod samples/livepatch/livepatch-callbacks-mod.ko
+ *       insmod: ERROR: could not insert module samples/livepatch/livepatch-callbacks-mod.ko: No such device
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/livepatch.h>
+#include <linux/stat.h>
+
+static int pre_patch_ret;
+module_param(pre_patch_ret, int, 0644);
+MODULE_PARM_DESC(pre_patch_ret, "pre_patch_ret (default=0)");
+
+const char *module_state[] = {
+	[MODULE_STATE_LIVE]	= "[MODULE_STATE_LIVE] Normal state",
+	[MODULE_STATE_COMING]	= "[MODULE_STATE_COMING] Full formed, running module_init",
+	[MODULE_STATE_GOING]	= "[MODULE_STATE_GOING] Going away",
+	[MODULE_STATE_UNFORMED]	= "[MODULE_STATE_UNFORMED] Still setting it up",
+};
+
+static void callback_info(const char *callback, struct klp_object *obj)
+{
+	if (obj->mod)
+		pr_info("%s: %s -> %s\n", callback, obj->mod->name,
+			module_state[obj->mod->state]);
+	else
+		pr_info("%s: vmlinux\n", callback);
+}
+
+/* Executed on object patching (ie, patch enablement) */
+static int pre_patch_callback(struct klp_object *obj)
+{
+	callback_info(__func__, obj);
+	return pre_patch_ret;
+}
+
+/* Executed on object unpatching (ie, patch disablement) */
+static void post_patch_callback(struct klp_object *obj)
+{
+	callback_info(__func__, obj);
+}
+
+/* Executed on object unpatching (ie, patch disablement) */
+static void pre_unpatch_callback(struct klp_object *obj)
+{
+	callback_info(__func__, obj);
+}
+
+/* Executed on object unpatching (ie, patch disablement) */
+static void post_unpatch_callback(struct klp_object *obj)
+{
+	callback_info(__func__, obj);
+}
+
+static struct klp_func funcs[] = {
+	{ }
+};
+
+static struct klp_object objs[] = {
+	{
+		.name = NULL,	/* vmlinux */
+		.funcs = funcs,
+		.callbacks = {
+			.pre_patch = pre_patch_callback,
+			.post_patch = post_patch_callback,
+			.pre_unpatch = pre_unpatch_callback,
+			.post_unpatch = post_unpatch_callback,
+		},
+	},	{
+		.name = "livepatch_callbacks_mod",
+		.funcs = funcs,
+		.callbacks = {
+			.pre_patch = pre_patch_callback,
+			.post_patch = post_patch_callback,
+			.pre_unpatch = pre_unpatch_callback,
+			.post_unpatch = post_unpatch_callback,
+		},
+	}, { }
+};
+
+static struct klp_patch patch = {
+	.mod = THIS_MODULE,
+	.objs = objs,
+};
+
+static int livepatch_callbacks_demo_init(void)
+{
+	int ret;
+
+	if (!klp_have_reliable_stack() && !patch.immediate) {
+		/*
+		 * WARNING: Be very careful when using 'patch.immediate' in
+		 * your patches.  It's ok to use it for simple patches like
+		 * this, but for more complex patches which change function
+		 * semantics, locking semantics, or data structures, it may not
+		 * be safe.  Use of this option will also prevent removal of
+		 * the patch.
+		 *
+		 * See Documentation/livepatch/livepatch.txt for more details.
+		 */
+		patch.immediate = true;
+		pr_notice("The consistency model isn't supported for your architecture.  Bypassing safety mechanisms and applying the patch immediately.\n");
+	}
+
+	ret = klp_register_patch(&patch);
+	if (ret)
+		return ret;
+	ret = klp_enable_patch(&patch);
+	if (ret) {
+		WARN_ON(klp_unregister_patch(&patch));
+		return ret;
+	}
+	return 0;
+}
+
+static void livepatch_callbacks_demo_exit(void)
+{
+	WARN_ON(klp_unregister_patch(&patch));
+}
+
+module_init(livepatch_callbacks_demo_init);
+module_exit(livepatch_callbacks_demo_exit);
+MODULE_LICENSE("GPL");
+MODULE_INFO(livepatch, "Y");
diff --git a/samples/livepatch/livepatch-callbacks-mod.c b/samples/livepatch/livepatch-callbacks-mod.c
new file mode 100644
index 000000000000..d0f8657e19f3
--- /dev/null
+++ b/samples/livepatch/livepatch-callbacks-mod.c
@@ -0,0 +1,53 @@
+/*
+ * Copyright (C) 2017 Joe Lawrence <joe.lawrence@xxxxxxxxxx>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+/*
+ * livepatch-callbacks-mod.c - (un)patching callbacks demo support module
+ *
+ *
+ * Purpose
+ * -------
+ *
+ * Simple module to demonstrate livepatch (un)patching callbacks.
+ *
+ *
+ * Usage
+ * -----
+ *
+ * This module is not intended to be standalone.  See the "Usage"
+ * section of livepatch-callbacks-mod.c.
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+
+static int livepatch_callbacks_mod_init(void)
+{
+	pr_info("%s\n", __func__);
+	return 0;
+}
+
+static void livepatch_callbacks_mod_exit(void)
+{
+	pr_info("%s\n", __func__);
+}
+
+module_init(livepatch_callbacks_mod_init);
+module_exit(livepatch_callbacks_mod_exit);
+MODULE_LICENSE("GPL");
-- 
1.8.3.1