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

Du, Changbin
2018-01-14T05:52:07Z

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