Re: [PATCH] tracing: Fix memleak due to race between current_tracer and trace

[Zheng Yejian]

On 2023/8/8 17:29, Zheng Yejian wrote:
> Kmemleak report a leak in graph_trace_open():
>
>    unreferenced object 0xffff0040b95f4a00 (size 128):
>      comm "cat", pid 204981, jiffies 4301155872 (age 99771.964s)
>      hex dump (first 32 bytes):
>        e0 05 e7 b4 ab 7d 00 00 0b 00 01 00 00 00 00 00 .....}..........
>        f4 00 01 10 00 a0 ff ff 00 00 00 00 65 00 10 00 ............e...
>      backtrace:
>        [<000000005db27c8b>] kmem_cache_alloc_trace+0x348/0x5f0
>        [<000000007df90faa>] graph_trace_open+0xb0/0x344
>        [<00000000737524cd>] __tracing_open+0x450/0xb10
>        [<0000000098043327>] tracing_open+0x1a0/0x2a0
>        [<00000000291c3876>] do_dentry_open+0x3c0/0xdc0
>        [<000000004015bcd6>] vfs_open+0x98/0xd0
>        [<000000002b5f60c9>] do_open+0x520/0x8d0
>        [<00000000376c7820>] path_openat+0x1c0/0x3e0
>        [<00000000336a54b5>] do_filp_open+0x14c/0x324
>        [<000000002802df13>] do_sys_openat2+0x2c4/0x530
>        [<0000000094eea458>] __arm64_sys_openat+0x130/0x1c4
>        [<00000000a71d7881>] el0_svc_common.constprop.0+0xfc/0x394
>        [<00000000313647bf>] do_el0_svc+0xac/0xec
>        [<000000002ef1c651>] el0_svc+0x20/0x30
>        [<000000002fd4692a>] el0_sync_handler+0xb0/0xb4
>        [<000000000c309c35>] el0_sync+0x160/0x180
>
> The root cause is descripted as follows:
>
>    __tracing_open() {  // 1. File 'trace' is being opened;
>      ...
>      *iter->trace = *tr->current_trace;  // 2. Tracer 'function_graph' is
>                                          //    currently set;
>      ...
>      iter->trace->open(iter);  // 3. Call graph_trace_open() here,
>                                //    and memory are allocated in it;
>      ...
>    }
>
>    s_start() {  // 4. The opened file is being read;
>      ...
>      *iter->trace = *tr->current_trace;  // 5. If tracer is switched to
>                                          //    'nop' or others, then memory
>                                          //    in step 3 are leaked!!!
>      ...
>    }
>
> To fix it, in s_start(), close tracer before switching then reopen the
> new tracer after switching.
>
> Fixes: d7350c3f4569 ("tracing/core: make the read callbacks reentrants")
> Signed-off-by: Zheng Yejian <zhengyejian1@xxxxxxxxxx>
> ---
>   kernel/trace/trace.c | 9 ++++++++-
>   1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index b8870078ef58..d50a0227baa3 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -4213,8 +4213,15 @@ static void *s_start(struct seq_file *m, loff_t *pos)
>   	 * will point to the same string as current_trace->name.
>   	 */
>   	mutex_lock(&trace_types_lock);
> -	if (unlikely(tr->current_trace && iter->trace->name != tr->current_trace->name))
> +	if (unlikely(tr->current_trace && iter->trace->name != tr->current_trace->name)) {
> +		/* Close iter->trace before switching to the new current tracer */
> +		if (iter->trace->close)
> +			iter->trace->close(iter);

Hi, kasan report an issue related to this patch in v5.10, I'll handle
it and then send V2 if the patch needs to be changed.

  BUG: KASAN: use-after-free in graph_trace_close+0x78/0x7c
  Read of size 8 at addr ffff204054ca8f00 by task cat/193541
  [...]
  Call trace:
   dump_backtrace+0x0/0x3e4
   show_stack+0x20/0x2c
   dump_stack+0x140/0x198
   print_address_description.constprop.0+0x2c/0x1fc
   __kasan_report+0xe0/0x140
   kasan_report+0x44/0x5c
   __asan_report_load8_noabort+0x34/0x60
   graph_trace_close+0x78/0x7c
   wakeup_trace_close+0x3c/0x54
   s_start+0x4f4/0x794
   seq_read_iter+0x210/0xd90
   seq_read+0x288/0x410
   vfs_read+0x13c/0x41c
   ksys_read+0xf4/0x1e0
   __arm64_sys_read+0x74/0xa4
   el0_svc_common.constprop.0+0xfc/0x394
   do_el0_svc+0xac/0xec
   el0_svc+0x20/0x30
   el0_sync_handler+0xb0/0xb4
   el0_sync+0x160/0x180

  Allocated by task 193541:
   kasan_save_stack+0x28/0x60
   __kasan_kmalloc.constprop.0+0xa4/0xd0
   kasan_kmalloc+0x10/0x20
   kmem_cache_alloc_trace+0x2ec/0x5f0
   graph_trace_open+0xb0/0x344
   __tracing_open+0x450/0xb10
   tracing_open+0x1a0/0x2a0
   do_dentry_open+0x3c0/0xdc0
   vfs_open+0x98/0xd0
   do_open+0x520/0x8d0

>   		*iter->trace = *tr->current_trace;
> +		/* Reopen the new current tracer */
> +		if (iter->trace->open)
> +			iter->trace->open(iter);
> +	}
>   	mutex_unlock(&trace_types_lock);
>   
>   #ifdef CONFIG_TRACER_MAX_TRACE