[PATCH] tracing/synthetic: Fix races on freeing last_cmd - Linux-stable-mirror

21 Mar 2023

From: "Tze-nan Wu" Tze-nan.Wu@mediatek.com
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
        ...
Fixes: 27c888da9867 ("tracing: Remove size restriction on synthetic event cmd error logging")
Cc: stable@vger.kernel.org
Signed-off-by: Tze-nan Wu Tze-nan.Wu@mediatek.com
---
 kernel/trace/trace_events_synth.c | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/kernel/trace/trace_events_synth.c b/kernel/trace/trace_events_synth.c
index 46d0abb32d0f..f0ff730125bf 100644
--- a/kernel/trace/trace_events_synth.c
+++ b/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)
 {
+	int ret = 0;
+
+	mutex_lock(&lastcmd_mutex);
    if (!str || !last_cmd)
-		return 0;
+		goto out;
-	return err_pos(last_cmd, str);
+	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);
-- 
2.18.0


    