linux-kernel - Re: [resend][PATCH] Added PR_SET_PROCTITLE

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Date:	Fri, 9 Oct 2009 22:57:14 -0400
From:	Bryan Donlan <bdonlan@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-kernel@...r.kernel.org, Ulrich Drepper <drepper@...hat.com>,
	linux-api@...r.kernel.org
Subject: Re: [resend][PATCH] Added PR_SET_PROCTITLE_AREA option for prctl()

On Fri, Oct 9, 2009 at 10:42 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:

>> >> + __ __ __ __ __ __ __ __ __ __ __ __ __ __ res += access_process_vm(task, mm->env_start,
>
> Your email client is converting tabs to non-ascii crap.  gmail.  Sigh.

Weird ... I'll have to see if I can do something about that :/

> OK.
>
> But there's no way in which the reader of either the patch or the
> resulting code can discover this subtlety.

I didn't write the log message or the code - I just mentioned these
same issues back in the lkml thread :) But yes, this should be
mentioned somewhere.

>> The solution is to use the seqlock to detect this, and prevent the
>> secret information from ever making it back to process B's userspace.
>> Note that it's not enough to just recheck arg_start, as process A may
>> reassign the proctitle area back to its original position after having
>> it somewhere else for a while.
>
> Well seqlock is _a_ solution.  Another is to use a mutex or an rwsem
> around the whole operation.
>
> With the code as you propose it, what happens if a process sits in a
> tight loop running setproctitle?  Do other processes running `ps' get
> stuck in a livelock until the offending process gets scheduled out?

It does seem like a maximum spin count should be put in there - and
maybe a timeout as well (since with FUSE etc it's possible to engineer
page faults that take arbitrarily long).
Also, it occurs to me that:

> +     do {
> +             seq = read_seqbegin(&mm->arg_lock);
> +
> +             len = mm->arg_end - mm->arg_start;
> +             if (len > PAGE_SIZE)
> +                     len = PAGE_SIZE;

If arg_end or arg_start are modified after this, is it truly safe to
assume that len will remain <= PAGE_SIZE without a memory barrier
before the conditional?
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