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 | 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);