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

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Date:	Tue, 13 Oct 2009 04:03:45 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Bryan Donlan <bdonlan@...il.com>
Cc:	kosaki.motohiro@...fujitsu.com,
	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()

Hi

Sorry for the delaying.

> 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?

Honestly, I don't worry about so much. if attacker want DoS attack, fork bomb is
efficient than this way. then, attacker never use this.


> 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...

Probably, I haven't understand this paragraph. Why is this FUSE related issue?

> 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...

Plus, please look my mesurement data as another post. seqlock implementation is very fast
although contention occured.



> >>> +     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...

Rgiht. I'll fix this issue at next spin.
Thank you.



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