[PATCH v6] kobject: Fix global-out-of-bounds in kobject_action_type()

[Xia Fukun]

The following c language code can trigger KASAN's global variable
out-of-bounds access error in kobject_action_type():

int main() {
    int fd;
    char *filename = "/sys/block/ram12/uevent";
    char str[86] = "offline";
    int len = 86;

    fd = open(filename, O_WRONLY);
    if (fd == -1) {
        printf("open");
        exit(1);
    }

    if (write(fd, str, len) == -1) {
        printf("write");
        exit(1);
    }

    close(fd);
    return 0;
}

Function kobject_action_type() receives the input parameters buf and count,
where count is the length of the string buf.

In the use case we provided, count is 86, the count_first is 85.
Buf points to a string with a length of 86, and its first seven
characters are "offline".
In line 87 of the code, kobject_actions[action] is the string "offline"
with the length of 7,an out-of-boundary access will appear:

kobject_actions[action][85].

Use sysfs_match_string() to replace the fragile and convoluted loop.
This function is well-tested for parsing sysfs inputs. Moreover, this
modification will not cause any functional changes.

Fixes: f36776fafbaa ("kobject: support passing in variables for synthetic uevents")
Signed-off-by: Xia Fukun <xiafukun@xxxxxxxxxx>
---
v5 -> v6:
- Ensure that the following extensions remain effective:
https://www.kernel.org/doc/Documentation/ABI/testing/sysfs-uevent

v4 -> v5:
- Fixed build errors and warnings, and retested the patch.

v3 -> v4:
- Refactor the function to be more obviously correct and readable.
---
 include/linux/kobject.h |  3 +++
 lib/kobject_uevent.c    | 30 +++++++++++++++++-------------
 2 files changed, 20 insertions(+), 13 deletions(-)

diff --git a/include/linux/kobject.h b/include/linux/kobject.h
index c392c811d9ad..9d3ecce3c4f6 100644
--- a/include/linux/kobject.h
+++ b/include/linux/kobject.h
@@ -32,6 +32,9 @@
 #define UEVENT_NUM_ENVP			64	/* number of env pointers */
 #define UEVENT_BUFFER_SIZE		2048	/* buffer for the variables */
 
+/* the maximum length of the string contained in kobject_actions[] */
+#define UEVENT_KACT_STRSIZE		16
+
 #ifdef CONFIG_UEVENT_HELPER
 /* path to the userspace helper executed on an event */
 extern char uevent_helper[];
diff --git a/lib/kobject_uevent.c b/lib/kobject_uevent.c
index 7c44b7ae4c5c..4030a928e9c6 100644
--- a/lib/kobject_uevent.c
+++ b/lib/kobject_uevent.c
@@ -66,7 +66,8 @@ static int kobject_action_type(const char *buf, size_t count,
 	enum kobject_action action;
 	size_t count_first;
 	const char *args_start;
-	int ret = -EINVAL;
+	int i, ret = -EINVAL;
+	char kobj_act_buf[UEVENT_KACT_STRSIZE] = "";
 
 	if (count && (buf[count-1] == '\n' || buf[count-1] == '\0'))
 		count--;
@@ -77,21 +78,24 @@ static int kobject_action_type(const char *buf, size_t count,
 	args_start = strnchr(buf, count, ' ');
 	if (args_start) {
 		count_first = args_start - buf;
+		if (count_first > UEVENT_KACT_STRSIZE)
+			goto out;
+
 		args_start = args_start + 1;
+		strncpy(kobj_act_buf, buf, count_first);
+		i = sysfs_match_string(kobject_actions, kobj_act_buf);
 	} else
-		count_first = count;
+		i = sysfs_match_string(kobject_actions, buf);
 
-	for (action = 0; action < ARRAY_SIZE(kobject_actions); action++) {
-		if (strncmp(kobject_actions[action], buf, count_first) != 0)
-			continue;
-		if (kobject_actions[action][count_first] != '\0')
-			continue;
-		if (args)
-			*args = args_start;
-		*type = action;
-		ret = 0;
-		break;
-	}
+	if (i < 0)
+		goto out;
+
+	action = i;
+	if (args)
+		*args = args_start;
+
+	*type = action;
+	ret = 0;
 out:
 	return ret;
 }
-- 
2.17.1