livepatch: introduce shadow variable API

Add exported API for livepatch modules:

  klp_shadow_get()
  klp_shadow_alloc()
  klp_shadow_get_or_alloc()
  klp_shadow_free()
  klp_shadow_free_all()

that implement "shadow" variables, which allow callers to associate new
shadow fields to existing data structures.  This is intended to be used
by livepatch modules seeking to emulate additions to data structure
definitions.

See Documentation/livepatch/shadow-vars.txt for a summary of the new
shadow variable API, including a few common use cases.

See samples/livepatch/livepatch-shadow-* for example modules that
demonstrate shadow variables.

[jkosina@suse.cz: fix __klp_shadow_get_or_alloc() comment as spotted by
 Josh]
Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

1 files changed, 168 insertions, 0 deletions

diff --git a/samples/livepatch/livepatch-shadow-fix2.c b/samples/livepatch/livepatch-shadow-fix2.c
new file mode 100644
index 000000000000..53c1794bdc5f
--- /dev/null
+++ b/samples/livepatch/livepatch-shadow-fix2.c
@@ -0,0 +1,168 @@
+/*
+ * Copyright (C) 2017 Joe Lawrence <joe.lawrence@redhat.com>
+ *
+ * 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-shadow-fix2.c - Shadow variables, livepatch demo
+ *
+ * Purpose
+ * -------
+ *
+ * Adds functionality to livepatch-shadow-mod's in-flight data
+ * structures through a shadow variable.  The livepatch patches a
+ * routine that periodically inspects data structures, incrementing a
+ * per-data-structure counter, creating the counter if needed.
+ *
+ *
+ * Usage
+ * -----
+ *
+ * This module is not intended to be standalone.  See the "Usage"
+ * section of livepatch-shadow-mod.c.
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/livepatch.h>
+#include <linux/slab.h>
+
+/* Shadow variable enums */
+#define SV_LEAK		1
+#define SV_COUNTER	2
+
+struct dummy {
+	struct list_head list;
+	unsigned long jiffies_expire;
+};
+
+bool livepatch_fix2_dummy_check(struct dummy *d, unsigned long jiffies)
+{
+	int *shadow_count;
+	int count;
+
+	/*
+	 * Patch: handle in-flight dummy structures, if they do not
+	 * already have a SV_COUNTER shadow variable, then attach a
+	 * new one.
+	 */
+	count = 0;
+	shadow_count = klp_shadow_get_or_alloc(d, SV_COUNTER,
+					       &count, sizeof(count),
+					       GFP_NOWAIT);
+	if (shadow_count)
+		*shadow_count += 1;
+
+	return time_after(jiffies, d->jiffies_expire);
+}
+
+void livepatch_fix2_dummy_free(struct dummy *d)
+{
+	void **shadow_leak, *leak;
+	int *shadow_count;
+
+	/* Patch: copy the memory leak patch from the fix1 module. */
+	shadow_leak = klp_shadow_get(d, SV_LEAK);
+	if (shadow_leak) {
+		leak = *shadow_leak;
+		klp_shadow_free(d, SV_LEAK);
+		kfree(leak);
+		pr_info("%s: dummy @ %p, prevented leak @ %p\n",
+			 __func__, d, leak);
+	} else {
+		pr_info("%s: dummy @ %p leaked!\n", __func__, d);
+	}
+
+	/*
+	 * Patch: fetch the SV_COUNTER shadow variable and display
+	 * the final count.  Detach the shadow variable.
+	 */
+	shadow_count = klp_shadow_get(d, SV_COUNTER);
+	if (shadow_count) {
+		pr_info("%s: dummy @ %p, check counter = %d\n",
+			__func__, d, *shadow_count);
+		klp_shadow_free(d, SV_COUNTER);
+	}
+
+	kfree(d);
+}
+
+static struct klp_func funcs[] = {
+	{
+		.old_name = "dummy_check",
+		.new_func = livepatch_fix2_dummy_check,
+	},
+	{
+		.old_name = "dummy_free",
+		.new_func = livepatch_fix2_dummy_free,
+	}, { }
+};
+
+static struct klp_object objs[] = {
+	{
+		.name = "livepatch_shadow_mod",
+		.funcs = funcs,
+	}, { }
+};
+
+static struct klp_patch patch = {
+	.mod = THIS_MODULE,
+	.objs = objs,
+};
+
+static int livepatch_shadow_fix2_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_shadow_fix2_exit(void)
+{
+	/* Cleanup any existing SV_COUNTER shadow variables */
+	klp_shadow_free_all(SV_COUNTER);
+
+	WARN_ON(klp_unregister_patch(&patch));
+}
+
+module_init(livepatch_shadow_fix2_init);
+module_exit(livepatch_shadow_fix2_exit);
+MODULE_LICENSE("GPL");
+MODULE_INFO(livepatch, "Y");