tracing/synthetic: Fix races on freeing last_cmd

Currently, the "last_cmd" variable can be accessed by multiple processes
asynchronously when multiple users manipulate synthetic_events node
at the same time, it could lead to use-after-free or double-free.

This patch add "lastcmd_mutex" to prevent "last_cmd" from being accessed
asynchronously.

================================================================

It's easy to reproduce in the KASAN environment by running the two
scripts below in different shells.

script 1:
        while :
        do
                echo -n -e '\x88' > /sys/kernel/tracing/synthetic_events
        done

script 2:
        while :
        do
                echo -n -e '\xb0' > /sys/kernel/tracing/synthetic_events
        done

================================================================
double-free scenario:

    process A                       process B
-------------------               ---------------
1.kstrdup last_cmd
                                  2.free last_cmd
3.free last_cmd(double-free)

================================================================
use-after-free scenario:

    process A                       process B
-------------------               ---------------
1.kstrdup last_cmd
                                  2.free last_cmd
3.tracing_log_err(use-after-free)

================================================================

Appendix 1. KASAN report double-free:

BUG: KASAN: double-free in kfree+0xdc/0x1d4
Free of addr ***** by task sh/4879
Call trace:
        ...
        kfree+0xdc/0x1d4
        create_or_delete_synth_event+0x60/0x1e8
        trace_parse_run_command+0x2bc/0x4b8
        synth_events_write+0x20/0x30
        vfs_write+0x200/0x830
        ...

Allocated by task 4879:
        ...
        kstrdup+0x5c/0x98
        create_or_delete_synth_event+0x6c/0x1e8
        trace_parse_run_command+0x2bc/0x4b8
        synth_events_write+0x20/0x30
        vfs_write+0x200/0x830
        ...

Freed by task 5464:
        ...
        kfree+0xdc/0x1d4
        create_or_delete_synth_event+0x60/0x1e8
        trace_parse_run_command+0x2bc/0x4b8
        synth_events_write+0x20/0x30
        vfs_write+0x200/0x830
        ...

================================================================
Appendix 2. KASAN report use-after-free:

BUG: KASAN: use-after-free in strlen+0x5c/0x7c
Read of size 1 at addr ***** by task sh/5483
sh: CPU: 7 PID: 5483 Comm: sh
        ...
        __asan_report_load1_noabort+0x34/0x44
        strlen+0x5c/0x7c
        tracing_log_err+0x60/0x444
        create_or_delete_synth_event+0xc4/0x204
        trace_parse_run_command+0x2bc/0x4b8
        synth_events_write+0x20/0x30
        vfs_write+0x200/0x830
        ...

Allocated by task 5483:
        ...
        kstrdup+0x5c/0x98
        create_or_delete_synth_event+0x80/0x204
        trace_parse_run_command+0x2bc/0x4b8
        synth_events_write+0x20/0x30
        vfs_write+0x200/0x830
        ...

Freed by task 5480:
        ...
        kfree+0xdc/0x1d4
        create_or_delete_synth_event+0x74/0x204
        trace_parse_run_command+0x2bc/0x4b8
        synth_events_write+0x20/0x30
        vfs_write+0x200/0x830
        ...

Link: https://lore.kernel.org/linux-trace-kernel/20230321110444.1587-1-Tze-nan.Wu@mediatek.com

Fixes: 27c888da98 ("tracing: Remove size restriction on synthetic event cmd error logging")
Cc: stable@vger.kernel.org
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Cc: "Tom Zanussi" <zanussi@kernel.org>
Signed-off-by: Tze-nan Wu <Tze-nan.Wu@mediatek.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

kernel/trace/trace_events_synth.c

@@ -44,14 +44,21 @@ enum { ERRORS };
 
 static const char *err_text[] = { ERRORS };
 
+DEFINE_MUTEX(lastcmd_mutex);
 static char *last_cmd;
 
 static int errpos(const char *str)
 {
-	if (!str || !last_cmd)
-		return 0;
+	int ret = 0;
 
-	return err_pos(last_cmd, str);
+	mutex_lock(&lastcmd_mutex);
+	if (!str || !last_cmd)
+		goto out;
+
+	ret = err_pos(last_cmd, str);
+ out:
+	mutex_unlock(&lastcmd_mutex);
+	return ret;
 }
 
 static void last_cmd_set(const char *str)
@@ -59,18 +66,22 @@ static void last_cmd_set(const char *str)
 	if (!str)
 		return;
 
+	mutex_lock(&lastcmd_mutex);
 	kfree(last_cmd);
-
 	last_cmd = kstrdup(str, GFP_KERNEL);
+	mutex_unlock(&lastcmd_mutex);
 }
 
 static void synth_err(u8 err_type, u16 err_pos)
 {
+	mutex_lock(&lastcmd_mutex);
 	if (!last_cmd)
-		return;
+		goto out;
 
 	tracing_log_err(NULL, "synthetic_events", last_cmd, err_text,
 			err_type, err_pos);
+ out:
+	mutex_unlock(&lastcmd_mutex);
 }
 
 static int create_synth_event(const char *raw_command);