[RFC 3/5] livepatch: Allow to distinguish different version of system state changes

[Petr Mladek]

The atomic replace runs pre/post (un)install callbacks only from the new
livepatch. There are several reasons for this:

  + Simplicity: clear ordering of operations, no interactions between
	old and new callbacks.

  + Reliability: only new livepatch knows what changes can already be made
	by older livepatches and how to take over the state.

  + Testing: the atomic replace can be properly tested only when a newer
	livepatch is available. It might be too late to fix unwanted effect
	of callbacks from older	livepatches.

It might happen that an older change is not enough and the same system
state has to be modified another way. Different changes need to get
distinguished by a version number added to struct klp_state.

The version can also be used to prevent loading incompatible livepatches.
The check is done when the livepatch is enabled. The rules are:

  + Any completely new system state modification is allowed.

  + System state modifications with the same or higher version are allowed
    for already modified system states.

  + Cumulative livepatches must handle all system state modifications from
    already installed livepatches.

  + Non-cumulative livepatches are allowed to touch already modified
    system states.

Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
---
 include/linux/livepatch.h |  2 ++
 kernel/livepatch/core.c   |  8 ++++++++
 kernel/livepatch/state.c  | 40 +++++++++++++++++++++++++++++++++++++++-
 kernel/livepatch/state.h  |  9 +++++++++
 4 files changed, 58 insertions(+), 1 deletion(-)
 create mode 100644 kernel/livepatch/state.h

diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
index 591abdee30d7..8bc4c6cc3f3f 100644
--- a/include/linux/livepatch.h
+++ b/include/linux/livepatch.h
@@ -135,10 +135,12 @@ struct klp_object {
 /**
  * struct klp_state - state of the system modified by the livepatch
  * @id:		system state identifier (non zero)
+ * @version:	version of the change (non-zero)
  * @data:	custom data
  */
 struct klp_state {
 	int id;
+	int version;
 	void *data;
 };
 
diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index 24c4a13bd26c..614642719825 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -21,6 +21,7 @@
 #include <asm/cacheflush.h>
 #include "core.h"
 #include "patch.h"
+#include "state.h"
 #include "transition.h"
 
 /*
@@ -1003,6 +1004,13 @@ int klp_enable_patch(struct klp_patch *patch)
 
 	mutex_lock(&klp_mutex);
 
+	if(!klp_is_patch_compatible(patch)) {
+		pr_err("Livepatch patch (%s) is not compatible with the already installed livepatches.\n",
+			patch->mod->name);
+		mutex_unlock(&klp_mutex);
+		return -EINVAL;
+	}
+
 	ret = klp_init_patch_early(patch);
 	if (ret) {
 		mutex_unlock(&klp_mutex);
diff --git a/kernel/livepatch/state.c b/kernel/livepatch/state.c
index f8822b71f96e..b54a69b9e4b4 100644
--- a/kernel/livepatch/state.c
+++ b/kernel/livepatch/state.c
@@ -12,7 +12,9 @@
 #include "transition.h"
 
 #define klp_for_each_state(patch, state)		\
-	for (state = patch->states; state && state->id; state++)
+	for (state = patch->states;			\
+	     state && state->id && state->version;	\
+	     state++)
 
 /**
  * klp_get_state() - get information about system state modified by
@@ -81,3 +83,39 @@ struct klp_state *klp_get_prev_state(int id)
 	return last_state;
 }
 EXPORT_SYMBOL_GPL(klp_get_prev_state);
+
+/* Check if the patch is able to deal with the given system state. */
+static bool klp_is_state_compatible(struct klp_patch *patch,
+				    struct klp_state *state)
+{
+	struct klp_state *new_state;
+
+	new_state = klp_get_state(patch, state->id);
+
+	if (new_state)
+		return new_state->version < state->version ? false : true;
+
+	/* Cumulative livepatch must handle all already modified states. */
+	return patch->replace ? false : true;
+}
+
+/*
+ * Check if the new livepatch will not break the existing system states.
+ * Cumulative patches must handle all already modified states.
+ * Non-cumulative patches can touch already modified states.
+ */
+bool klp_is_patch_compatible(struct klp_patch *patch)
+{
+	struct klp_patch *old_patch;
+	struct klp_state *state;
+
+
+	klp_for_each_patch(old_patch) {
+		klp_for_each_state(old_patch, state) {
+			if (!klp_is_state_compatible(patch, state))
+				return false;
+		}
+	}
+
+	return true;
+}
diff --git a/kernel/livepatch/state.h b/kernel/livepatch/state.h
new file mode 100644
index 000000000000..49d9c16e8762
--- /dev/null
+++ b/kernel/livepatch/state.h
@@ -0,0 +1,9 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LIVEPATCH_STATE_H
+#define _LIVEPATCH_STATE_H
+
+#include <linux/livepatch.h>
+
+bool klp_is_patch_compatible(struct klp_patch *patch);
+
+#endif /* _LIVEPATCH_STATE_H */
-- 
2.16.4