[PATCH 3/8] livepatch: Add atomic replace

[Petr Mladek]

From: Jason Baron <jbaron@xxxxxxxxxx>

Sometimes we would like to revert a particular fix. Currently, this
is not easy because we want to keep all other fixes active and we
could revert only the last applied patch.

One solution would be to apply new patch that implemented all
the reverted functions like in the original code. It would work
as expected but there will be unnecessary redirections. In addition,
it would also require knowing which functions need to be reverted at
build time.

Another problem is when there are many patches that touch the same
functions. There might be dependencies between patches that are
not enforced on the kernel side. Also it might be pretty hard to
actually prepare the patch and ensure compatibility with the other
patches.

A better solution would be to create cumulative patch and say that
it replaces all older ones.

This patch adds a new "replace" flag to struct klp_patch. When it is
enabled, a set of 'nop' klp_func will be dynamically created for all
functions that are already being patched but that will no longer be
modified by the new patch. They used as a new target during the patch
transition.

The idea is to handle Nops' structures like the static ones. When
the dynamic structures are allocated, we initialize all values that
are normally statically defined. The only exception is "new_func"
in struct klp_func. It has to point to the original function. But
the address is known only when the object (module) is loaded.

Nevertheless we still need to distinguish the dynamically allocated
structures in some operations. For this, we add "nop" flag into
struct klp_func and "dynamic" flag into struct klp_object. They
need special handling in the following situations:

  + The structures are added into the lists of objects and functions
    immediately. In fact, the lists were created for this purpose.

  + The address of the original function is known only when the patched
    object (module) is loaded. Therefore it is copied later in
    klp_init_object_loaded().

  + The ftrace handler must not set PC to func->new_addr. It would cause
    infinite loop because the address points back to the beginning of
    the original function.

  + The various free() functions must free the structure itself.

Note that other ways to detect the dynamic structures are not considered
safe. For example, even the statically defined struct klp_object might
include empty funcs array. It might be there just to run some callbacks.

Special callbacks handling:

The callbacks from the replaced patches are _not_ called by intention.
It would be pretty hard to define a reasonable semantic and implement it.

It might even be counter-productive. The new patch is cumulative. It is
supposed to include most of the changes from older patches. In most cases,
it will not want to call pre_unpatch() post_unpatch() callbacks from
the replaced patches. It would disable/break things for no good reasons.
Also it should be easier to handle various scenarios in a single script
in the new patch than think about interactions caused by running many
scripts from older patches. Not to say that the old scripts even would
not expect to be called in this situation.

Signed-off-by: Jason Baron <jbaron@xxxxxxxxxx>
[pmladek@xxxxxxxx: Split, reuse existing code, simplified]
Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Jessica Yu <jeyu@xxxxxxxxxx>
Cc: Jiri Kosina <jikos@xxxxxxxxxx>
Cc: Miroslav Benes <mbenes@xxxxxxx>
---
 include/linux/livepatch.h |   6 ++
 kernel/livepatch/core.c   | 193 +++++++++++++++++++++++++++++++++++++++++++++-
 kernel/livepatch/patch.c  |   8 ++
 3 files changed, 204 insertions(+), 3 deletions(-)

diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
index f0a5a28b1386..f28af280f9e0 100644
--- a/include/linux/livepatch.h
+++ b/include/linux/livepatch.h
@@ -47,6 +47,7 @@
  * @stack_node:	list node for klp_ops func_stack list
  * @old_size:	size of the old function
  * @new_size:	size of the new function
+ * @nop:        temporary patch to use the original code again; dyn. allocated
  * @patched:	the func has been added to the klp_ops list
  * @transition:	the func is currently being applied or reverted
  *
@@ -84,6 +85,7 @@ struct klp_func {
 	struct list_head node;
 	struct list_head stack_node;
 	unsigned long old_size, new_size;
+	bool nop;
 	bool patched;
 	bool transition;
 };
@@ -122,6 +124,7 @@ struct klp_callbacks {
  *		(NULL for vmlinux)
  * @func_list:	dynamic list of the function entries
  * @node:	list node for klp_patch obj_list
+ * @dynamic:    temporary object for nop functions; dynamically allocated
  * @patched:	the object's funcs have been added to the klp_ops list
  */
 struct klp_object {
@@ -135,6 +138,7 @@ struct klp_object {
 	struct list_head func_list;
 	struct list_head node;
 	struct module *mod;
+	bool dynamic;
 	bool patched;
 };
 
@@ -142,6 +146,7 @@ struct klp_object {
  * struct klp_patch - patch structure for live patching
  * @mod:	reference to the live patch module
  * @objs:	object entries for kernel objects to be patched
+ * @replace:	replace all already registered patches
  * @list:	list node for global list of registered patches
  * @kobj:	kobject for sysfs resources
  * @obj_list:	dynamic list of the object entries
@@ -152,6 +157,7 @@ struct klp_patch {
 	/* external */
 	struct module *mod;
 	struct klp_object *objs;
+	bool replace;
 
 	/* internal */
 	struct list_head list;
diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index dcce028ecde8..18c400bd9a33 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -113,6 +113,40 @@ static bool klp_initialized(void)
 	return !!klp_root_kobj;
 }
 
+static struct klp_func *klp_find_func(struct klp_object *obj,
+				      struct klp_func *old_func)
+{
+	struct klp_func *func;
+
+	klp_for_each_func(obj, func) {
+		if ((strcmp(old_func->old_name, func->old_name) == 0) &&
+		    (old_func->old_sympos == func->old_sympos)) {
+			return func;
+		}
+	}
+
+	return NULL;
+}
+
+static struct klp_object *klp_find_object(struct klp_patch *patch,
+					  struct klp_object *old_obj)
+{
+	struct klp_object *obj;
+
+	klp_for_each_object(patch, obj) {
+		if (klp_is_module(old_obj)) {
+			if (klp_is_module(obj) &&
+			    strcmp(old_obj->name, obj->name) == 0) {
+				return obj;
+			}
+		} else if (!klp_is_module(obj)) {
+			return obj;
+		}
+	}
+
+	return NULL;
+}
+
 struct klp_find_arg {
 	const char *objname;
 	const char *name;
@@ -610,6 +644,123 @@ static struct attribute *klp_patch_attrs[] = {
 	NULL
 };
 
+/*
+ * Dynamically allocated objects and functions.
+ */
+static void klp_free_object_dynamic(struct klp_object *obj)
+{
+	kfree(obj->name);
+	kfree(obj);
+}
+
+static struct klp_object *klp_alloc_object_dynamic(const char *name)
+{
+	struct klp_object *obj;
+
+	obj = kzalloc(sizeof(*obj), GFP_KERNEL);
+	if (!obj)
+		return NULL;
+
+	if (name) {
+		obj->name = kstrdup(name, GFP_KERNEL);
+		if (!obj->name) {
+			kfree(obj);
+			return NULL;
+		}
+	}
+
+	INIT_LIST_HEAD(&obj->func_list);
+	obj->dynamic = true;
+
+	return obj;
+}
+
+static void klp_free_func_nop(struct klp_func *func)
+{
+	kfree(func->old_name);
+	kfree(func);
+}
+
+static struct klp_func *klp_alloc_func_nop(struct klp_func *old_func,
+					   struct klp_object *obj)
+{
+	struct klp_func *func;
+
+	func = kzalloc(sizeof(*func), GFP_KERNEL);
+	if (!func)
+		return NULL;
+
+	if (old_func->old_name) {
+		func->old_name = kstrdup(old_func->old_name, GFP_KERNEL);
+		if (!func->old_name) {
+			kfree(func);
+			return NULL;
+		}
+	}
+
+	/*
+	 * func->new_func is same as func->old_addr. These addresses are
+	 * set when the object is loaded, see klp_init_object_loaded().
+	 */
+	func->old_sympos = old_func->old_sympos;
+	func->nop = true;
+
+	return func;
+}
+
+static int klp_add_object_nops(struct klp_patch *patch,
+			       struct klp_object *old_obj)
+{
+	struct klp_object *obj;
+	struct klp_func *func, *old_func;
+
+	obj = klp_find_object(patch, old_obj);
+
+	if (!obj) {
+		obj = klp_alloc_object_dynamic(old_obj->name);
+		if (!obj)
+			return -ENOMEM;
+
+		list_add(&obj->node, &patch->obj_list);
+	}
+
+	klp_for_each_func(old_obj, old_func) {
+		func = klp_find_func(obj, old_func);
+		if (func)
+			continue;
+
+		func = klp_alloc_func_nop(old_func, obj);
+		if (!func)
+			return -ENOMEM;
+
+		list_add(&func->node, &obj->func_list);
+	}
+
+	return 0;
+}
+
+/*
+ * Add 'nop' functions which simply return to the caller to run
+ * the original function. The 'nop' functions are added to a
+ * patch to facilitate a 'replace' mode.
+ */
+static int klp_add_nops(struct klp_patch *patch)
+{
+	struct klp_patch *old_patch;
+	struct klp_object *old_obj;
+	int err = 0;
+
+	list_for_each_entry(old_patch, &klp_patches, list) {
+		klp_for_each_object(old_patch, old_obj) {
+			err = klp_add_object_nops(patch, old_obj);
+			if (err)
+				return err;
+		}
+	}
+
+	return 0;
+}
+
 static void klp_kobj_release_patch(struct kobject *kobj)
 {
 	struct klp_patch *patch;
@@ -626,6 +777,12 @@ static struct kobj_type klp_ktype_patch = {
 
 static void klp_kobj_release_object(struct kobject *kobj)
 {
+	struct klp_object *obj;
+
+	obj = container_of(kobj, struct klp_object, kobj);
+
+	if (obj->dynamic)
+		klp_free_object_dynamic(obj);
 }
 
 static struct kobj_type klp_ktype_object = {
@@ -635,6 +792,12 @@ static struct kobj_type klp_ktype_object = {
 
 static void klp_kobj_release_func(struct kobject *kobj)
 {
+	struct klp_func *func;
+
+	func = container_of(kobj, struct klp_func, kobj);
+
+	if (func->nop)
+		klp_free_func_nop(func);
 }
 
 static struct kobj_type klp_ktype_func = {
@@ -650,6 +813,8 @@ static void klp_free_funcs(struct klp_object *obj)
 		/* Might be called from klp_init_patch() error path. */
 		if (func->kobj.state_initialized)
 			kobject_put(&func->kobj);
+		else if (func->nop)
+			klp_free_func_nop(func);
 	}
 }
 
@@ -660,8 +825,12 @@ static void klp_free_object_loaded(struct klp_object *obj)
 
 	obj->mod = NULL;
 
-	klp_for_each_func(obj, func)
+	klp_for_each_func(obj, func) {
 		func->old_addr = 0;
+
+		if (func->nop)
+			func->new_func = NULL;
+	}
 }
 
 static void klp_free_objects(struct klp_patch *patch)
@@ -674,6 +843,8 @@ static void klp_free_objects(struct klp_patch *patch)
 		/* Might be called from klp_init_patch() error path. */
 		if (obj->kobj.state_initialized)
 			kobject_put(&obj->kobj);
+		else if (obj->dynamic)
+			klp_free_object_dynamic(obj);
 	}
 }
 
@@ -687,7 +858,14 @@ static void klp_free_patch(struct klp_patch *patch)
 
 static int klp_init_func(struct klp_object *obj, struct klp_func *func)
 {
-	if (!func->old_name || !func->new_func)
+	if (!func->old_name)
+		return -EINVAL;
+
+	/*
+	 * NOPs get the address later. The the patched module must be loaded,
+	 * see klp_init_object_loaded().
+	 */
+	if (!func->new_func && !func->nop)
 		return -EINVAL;
 
 	INIT_LIST_HEAD(&func->stack_node);
@@ -742,6 +920,9 @@ static int klp_init_object_loaded(struct klp_patch *patch,
 			return -ENOENT;
 		}
 
+		if (func->nop)
+			func->new_func = (void *)func->old_addr;
+
 		ret = kallsyms_lookup_size_offset((unsigned long)func->new_func,
 						  &func->new_size, NULL);
 		if (!ret) {
@@ -760,7 +941,7 @@ static int klp_init_object(struct klp_patch *patch, struct klp_object *obj)
 	int ret;
 	const char *name;
 
-	if (!obj->funcs)
+	if (!obj->funcs && !obj->dynamic)
 		return -EINVAL;
 
 	obj->patched = false;
@@ -810,6 +991,12 @@ static int klp_init_patch(struct klp_patch *patch)
 		return ret;
 	}
 
+	if (patch->replace) {
+		ret = klp_add_nops(patch);
+		if (ret)
+			goto free;
+	}
+
 	klp_for_each_object(patch, obj) {
 		ret = klp_init_object(patch, obj);
 		if (ret)
diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c
index 82d584225dc6..fbf1a3a47fc3 100644
--- a/kernel/livepatch/patch.c
+++ b/kernel/livepatch/patch.c
@@ -118,7 +118,15 @@ static void notrace klp_ftrace_handler(unsigned long ip,
 		}
 	}
 
+	/*
+	 * NOPs are used to replace existing patches with original code.
+	 * Do nothing! Setting pc would cause an infinite loop.
+	 */
+	if (func->nop)
+		goto unlock;
+
 	klp_arch_set_pc(regs, (unsigned long)func->new_func);
+
 unlock:
 	preempt_enable_notrace();
 }
-- 
2.13.6