Re: [PATCH] tracing: Fix potential null-pointer-access of entry in list 'tr->err_log'

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Mon, 14 Nov 2022 18:46:32 +0800
Zheng Yejian <zhengyejian1@huawei.com> wrote:

> Entries in list 'tr->err_log' will be reused after entry number
> exceed TRACING_LOG_ERRS_MAX.
> 
> The cmd string of the to be reused entry will be freed first then
> allocated a new one. If the allocation failed, then the entry will
> still be in list 'tr->err_log' but its 'cmd' field is set to be NULL,
> later access of 'cmd' is risky.
> 
> Currently above problem can cause the loss of 'cmd' information of first
> entry in 'tr->err_log'. When execute `cat /sys/kernel/tracing/error_log`,
> reproduce logs like:
>   [   37.495100] trace_kprobe: error: Maxactive is not for kprobe(null) ^
>   [   38.412517] trace_kprobe: error: Maxactive is not for kprobe
>     Command: p4:myprobe2 do_sys_openat2
>             ^

This looks good to me.

Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

BTW, I'm interested in how did you reproduce it. Did you inject a memory
allocation failure to reproduce it?

Thank you,

> 
> Fixes: 1581a884b7ca ("tracing: Remove size restriction on tracing_log_err cmd strings")
> Signed-off-by: Zheng Yejian <zhengyejian1@huawei.com>
> ---
>  kernel/trace/trace.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 47a44b055a1d..5ae776598106 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -7802,6 +7802,7 @@ static struct tracing_log_err *get_tracing_log_err(struct trace_array *tr,
>  						   int len)
>  {
>  	struct tracing_log_err *err;
> +	char *cmd;
>  
>  	if (tr->n_err_log_entries < TRACING_LOG_ERRS_MAX) {
>  		err = alloc_tracing_log_err(len);
> @@ -7810,12 +7811,12 @@ static struct tracing_log_err *get_tracing_log_err(struct trace_array *tr,
>  
>  		return err;
>  	}
> -
> +	cmd = kzalloc(len, GFP_KERNEL);
> +	if (!cmd)
> +		return ERR_PTR(-ENOMEM);
>  	err = list_first_entry(&tr->err_log, struct tracing_log_err, list);
>  	kfree(err->cmd);
> -	err->cmd = kzalloc(len, GFP_KERNEL);
> -	if (!err->cmd)
> -		return ERR_PTR(-ENOMEM);
> +	err->cmd = cmd;
>  	list_del(&err->list);
>  
>  	return err;
> -- 
> 2.25.1
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>