linux-kernel - Re: [PATCH linux-next] tracing: use strscpy() to instead of strncpy()

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Message-ID: <20230124121703.1e3a1ac4@gandalf.local.home>
Date:   Tue, 24 Jan 2023 12:17:03 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     <yang.yang29@....com.cn>
Cc:     <mhiramat@...nel.org>, <linux-kernel@...r.kernel.org>,
        <linux-trace-kernel@...r.kernel.org>, <xu.panda@....com.cn>
Subject: Re: [PATCH linux-next] tracing: use strscpy() to instead of
 strncpy()

On Mon, 9 Jan 2023 19:39:21 +0800 (CST)
<yang.yang29@....com.cn> wrote:

> From: Xu Panda <xu.panda@....com.cn>
> 
> The implementation of strscpy() is more robust and safer.
> That's now the recommended way to copy NUL-terminated strings.

But the string being copied is *not* NUL-terminated! And this change causes
a bug.

This is the 3rd patch I've seen that blindly converts strncpy() to
strscpy() and causes a bug in doing so. Not very safe if you ask me.

> 
> Signed-off-by: Xu Panda <xu.panda@....com.cn>
> Signed-off-by: Yang Yang <yang.yang29@....com.cn>
> ---
>  kernel/trace/trace_events_synth.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/kernel/trace/trace_events_synth.c b/kernel/trace/trace_events_synth.c
> index 67592eed0be8..cd636edd045e 100644
> --- a/kernel/trace/trace_events_synth.c
> +++ b/kernel/trace/trace_events_synth.c
> @@ -195,8 +195,7 @@ static int synth_field_string_size(char *type)
>  	if (len == 0)
>  		return 0; /* variable-length string */
> 
> -	strncpy(buf, start, len);
> -	buf[len] = '\0';
> +	strscpy(buf, start, len + 1);
> 
>  	err = kstrtouint(buf, 0, &size);
>  	if (err)


Here's the code being affected:

static int synth_field_string_size(char *type)
{
	char buf[4], *end, *start;
	unsigned int len;
	int size, err;

	start = strstr(type, "char[");
	if (start == NULL)
		return -EINVAL;
	start += sizeof("char[") - 1;

	end = strchr(type, ']');
	if (!end || end < start || type + strlen(type) > end + 1)
		return -EINVAL;

	len = end - start;
	if (len > 3)
		return -EINVAL;

	if (len == 0)
		return 0; /* variable-length string */

	strncpy(buf, start, len);
	buf[len] = '\0';

And you are replacing the above two lines with just:

	strscpy(buf, start, len + 1);


If you noticed, the string being placed into buf is:

  "char[123]"

Where we want to copy that "123" into buf.

strscpy() expects the source to be nul terminated, or it will return -E2BIG.

So the above will *always* return -E2BIG *and* not end buf[] with '\0' as
if strscpy() returns -E2BIG, then buf[] is not guaranteed to be
NUL-terminated.

  NACK!

-- Steve