[PATCH] tracing: Fix use-after-free and double-free on last_cmd

From: Cheng-Jui Wang
Date: Fri Mar 17 2023 - 01:31:27 EST


From: "Tze-nan Wu" <Tze-nan.Wu@xxxxxxxxxxxx>

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@xxxxxxxxxxxxxxx
Signed-off-by: Tze-nan Wu <Tze-nan.Wu@xxxxxxxxxxxx>
---
kernel/trace/trace_events_synth.c | 23 +++++++++++++++++++----
1 file changed, 19 insertions(+), 4 deletions(-)

diff --git a/kernel/trace/trace_events_synth.c b/kernel/trace/trace_events_synth.c
index 46d0abb32d0f..ce438eccab2e 100644
--- a/kernel/trace/trace_events_synth.c
+++ b/kernel/trace/trace_events_synth.c
@@ -42,16 +42,25 @@ enum { ERRORS };
#undef C
#define C(a, b) b

+static DEFINE_MUTEX(lastcmd_mutex);
+
static const char *err_text[] = { ERRORS };

static char *last_cmd;

static int errpos(const char *str)
{
- if (!str || !last_cmd)
- return 0;
+ int ret = 0;
+
+ mutex_lock(&lastcmd_mutex);
+ if (!str || !last_cmd) {
+ mutex_unlock(&lastcmd_mutex);
+ return ret;
+ }

- return err_pos(last_cmd, str);
+ ret = err_pos(last_cmd, str);
+ mutex_unlock(&lastcmd_mutex);
+ return ret;
}

static void last_cmd_set(const char *str)
@@ -59,18 +68,24 @@ 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)
{
- if (!last_cmd)
+ mutex_lock(&lastcmd_mutex);
+ if (!last_cmd) {
+ mutex_unlock(&lastcmd_mutex);
return;
+ }

tracing_log_err(NULL, "synthetic_events", last_cmd, err_text,
err_type, err_pos);
+ mutex_unlock(&lastcmd_mutex);
}

static int create_synth_event(const char *raw_command);
--
2.18.0