Re: [PATCH] tracing: Check return value of __create_val_fields() before using its result

[Tom Zanussi]

Hi Steve,

On Tue, 2020-10-13 at 15:48 -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>
> 
> After having a typo for writing a histogram trigger.
> 
> Wrote:
>   echo 'hist:key=pid:ts=common_timestamp.usec' >
> events/sched/sched_waking/trigger
> 
> Instead of:
>   echo 'hist:key=pid:ts=common_timestamp.usecs' >
> events/sched/sched_waking/trigger
> 
> and the following crash happened:
> 
>  BUG: kernel NULL pointer dereference, address: 0000000000000008
>  #PF: supervisor read access in kernel mode
>  #PF: error_code(0x0000) - not-present page
>  PGD 0 P4D 0
>  Oops: 0000 [#1] PREEMPT SMP PTI
>  CPU: 4 PID: 1641 Comm: sh Not tainted 5.9.0-rc5-test+ #549
>  Hardware name: Hewlett-Packard HP Compaq Pro 6300 SFF/339A, BIOS K01
> v03.03 07/14/2016
>  RIP: 0010:event_hist_trigger_func+0x70b/0x1ee0
>  Code: 24 08 89 d5 49 89 cc e9 8c 00 00 00 4c 89 f2 41 b9 00 10 00 00
> 4c 89 e1 44 89 ee 4c 89 ff e8 dc d3 ff ff 45 89 ea 4b 8b 14 d7 <f6>
> 42 08 04 74 17 41 8b 8f c0 00 00 00 8d 71 01 41 89 b7 c0 00 00
>  RSP: 0018:ffff959213d53db0 EFLAGS: 00010202
>  RAX: ffffffffffffffea RBX: 0000000000000000 RCX: 0000000000084c04
>  RDX: 0000000000000000 RSI: df7326aefebd174c RDI: 0000000000031080
>  RBP: 0000000000000002 R08: 0000000000000001 R09: 0000000000000001
>  R10: 0000000000000001 R11: 0000000000000046 R12: ffff959211dcf690
>  R13: 0000000000000001 R14: ffff95925a36e370 R15: ffff959251c89800
>  FS:  00007fb9ea934740(0000) GS:ffff95925ab00000(0000)
> knlGS:0000000000000000
>  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>  CR2: 0000000000000008 CR3: 00000000c976c005 CR4: 00000000001706e0
>  Call Trace:
>   ? trigger_process_regex+0x78/0x110
>   trigger_process_regex+0xc5/0x110
>   event_trigger_write+0x71/0xd0
>   vfs_write+0xca/0x210
>   ksys_write+0x70/0xf0
>   do_syscall_64+0x33/0x40
>   entry_SYSCALL_64_after_hwframe+0x44/0xa9
>  RIP: 0033:0x7fb9eaa29487
>  Code: 64 89 02 48 c7 c0 ff ff ff ff eb bb 0f 1f 80 00 00 00 00 f3 0f
> 1e fa 64 8b 04 25 18 00 00 00 85 c0 75 10 b8 01 00 00 00 0f 05 <48>
> 3d 00 f0 ff ff 77 51 c3 48 83 ec 28 48 89 54 24 18 48 89 74 24
> 
> This was caused by accessing the hlist_data fields after the call to
> __create_val_fields() without checking if the creation succeed.
> 
> Fixes: 63a1e5de3006 ("tracing: Save normal string variables")

Yes, this fixes the fix, sigh.

Reviewed-by: Tom Zanussi <zanussi@xxxxxxxxxx>

Thanks,

Tom


> Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
> ---
>  kernel/trace/trace_events_hist.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/trace_events_hist.c
> b/kernel/trace/trace_events_hist.c
> index c74a7d157306..96c3f86b81c5 100644
> --- a/kernel/trace/trace_events_hist.c
> +++ b/kernel/trace/trace_events_hist.c
> @@ -3687,7 +3687,7 @@ static int create_var_field(struct
> hist_trigger_data *hist_data,
>  
>  	ret = __create_val_field(hist_data, val_idx, file, var_name,
> expr_str, flags);
>  
> -	if (hist_data->fields[val_idx]->flags & HIST_FIELD_FL_STRING)
> +	if (!ret && hist_data->fields[val_idx]->flags &
> HIST_FIELD_FL_STRING)
>  		hist_data->fields[val_idx]->var_str_idx = hist_data-
> >n_var_str++;
>  
>  	return ret;