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

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Date:	Sat, 10 Oct 2009 03:11:48 -0400
From:	Bryan Donlan <bdonlan@...il.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Ulrich Drepper <drepper@...hat.com>,
	linux-api@...r.kernel.org, Timo Sirainen <tss@....fi>
Subject: Re: [resend][PATCH] Added PR_SET_PROCTITLE_AREA option for prctl()

On Sat, Oct 10, 2009 at 2:32 AM, KOSAKI Motohiro
<kosaki.motohiro@...fujitsu.com> wrote:

>> 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:
>
> makes sense.
> I like maximum spin rather than timeout.

I'm worried about the scenario where process A sets its cmdline buffer
to point to a page which will take a _VERY_ long time to pagein (maybe
forever), and then process B goes to try to read its cmdline. What
happens now?
Process A can arrange for this to happen by using a FUSE filesystem
that sits on a read forever. And since the first thing the admin's
likely to do to track down the problem is 'ps awux', this is liable to
be a rather nasty DoS...

Of course, this is no worse than it is now - it's already possible to
replace the page in question. But we should think about ways this
could be fixed for good...

>
>>> +     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?
>
> 1) access_process_vm() doesn't return error value.
> 2) read_seqretry(&mm->arg_lock, seq)) check seq, not mm->arg_start or len.
>
> then, if arg_{start,end} is modified, access_process_vm() may return 0
> and strnlen
> makes bad calculation, but read_seqretry() can detect its modify
> rightly. I think.

No, I'm worried about what if the compiler decides to rewrite like so:
if (mm->arg_end - mm->arg_start > PAGE_SIZE)
  len = PAGE_SIZE;
else /* here we reload arg_end/arg_start! */
  len = mm->arg_end - mm->arg_start;

Now we might write into buffer more than PAGE_SIZE bytes, which is
probably a buffer overrun into kernel space...
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