linux-kernel - Re: [PATCH 3/3] tracing: don't set parser->cont if it has reached the end of input buffer

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Date:   Sun, 14 Jan 2018 13:43:48 +0800
From:   "Du, Changbin" <changbin.du@...el.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     "Du, Changbin" <changbin.du@...el.com>, jolsa@...hat.com,
        peterz@...radead.org, mingo@...hat.com,
        alexander.shishkin@...ux.intel.com, linux-kernel@...r.kernel.org,
        linux-perf-users@...r.kernel.org
Subject: Re: [PATCH 3/3] tracing: don't set parser->cont if it has reached
 the end of input buffer

On Fri, Jan 12, 2018 at 10:31:08AM -0500, Steven Rostedt wrote:
[...]
> > Thanks, so now I unstand why below corner case. The userspace try to set the
> > filter with a unrecognized symbole name (e.g "abcdefg").
> > open("/sys/kernel/debug/tracing/set_ftrace_filter", O_WRONLY|O_TRUNC) = 3
> > write(3, "abcdefg", 7)
> > 
> > Since "abcdefg" is not in the symbole list, so we would expect the write return
> > -EINVAL, right? As below:
> > # echo abcdefg > set_ftrace_filter
> > bash: echo: write error: Invalid argument
> 
> The write itself doesn't finish the operation. There may be another
> write. In other words:
> 
> 	write(3, "do_", 3);
> 	write(3, "IRQ\n", 4);
> 
> Should both return success, even though it only enabled do_IRQ.
> 
> > 
> > But the above mechanism hide the error. It return success actually no filter is
> > apllied at all.
> > # echo -n abcdefg > set_ftrace_filter
> > 
> > I think in this case kernel may request the userspace append a '\0' or space to the
> > string buffer so everything can work.
> > 
> > Also there is another corner case. Below write dosn't work.
> > open("/sys/kernel/debug/tracing//set_ftrace_pid", O_WRONLY|O_TRUNC) = 3
> > write(3, " \0", 2)                      = -1 EINVAL (Invalid argument)
> > 
> > While these works:
> > # echo "" > set_ftrace_pid
> > # echo " " > set_ftrace_pid
> > # echo -n " " > set_ftrace_pid
> > 
> > These is the reason why I think '\0' should be recognized by the parser.
> 
> Hmm, thinking about this more, I do partially agree with you. We should
> accept '\0' but I disagree that it should be treated as a space. I
> don't want hidden code.
> 
> It should be treated as a terminator. And carefully as well.
> 
> 	write(3, "do_IRQ", 7);
> 
> Which will send to the kernel 'd' 'o' '_' 'I' 'R' 'Q' '\0' when the
> kernel sees the '\0', and the write has not sent anything else, it
> should go ahead and execute 'do_IRQ'
> 
> This will allow for this to work:
> 
> 	char *funcs[] = { "do_IRQ", "schedule", NULL };
> 
> 	for (i = 0; funcs[i]; i++) {
> 		ret = write(3, funcs[i], strlen(funcs[i]) + 1);
> 		if (ret < 0)
> 			exit(-1);
> 	}
> 
> 
> Now if someone were to write:
> 
> 	write(3, "do_IRQ\0schedule", 16);
> 
> That should return an error.
> 
> Why?
> 
> Because these are strings, and most tools treat '\0' as a nul
> terminator to a string. If we allow for tools to send data after that
> nul terminator, we are opening up a way for those interacting with
> these tools to sneak in strings that are not visible.
> 
> Say we have some admin tools that is doing tracing, and takes input.
> And all the input is logged. And say the tool does something like:
> 
> 
> 	r = read(0, buf, sizeof(buf));
> 	if (r < 0 || r > sizeof(buf) - 1)
> 		return -1;
> 	log("Adding to output %s\n", buf);
> 	write(3, buf, r);
> 
> The "Adding to output" would only show up to the '\0', but if we allow
> that write to process after the '\0' then we just allowed the user to
> circumvent the log.
> 
> -- Steve
I agree on your concern. So I will revise this serias and drop the last patch.

-- 
Thanks,
Changbin Du