linux-kernel - Re: [RFC v2 PATCH] futex: extend set_robust_list to allow 2 locking ABIs at the same time.

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Date:   Tue, 5 Nov 2019 15:48:33 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Florian Weimer <fweimer@...hat.com>
cc:     Carlos O'Donell <carlos@...hat.com>, Shawn Landden <shawn@....icu>,
        libc-alpha@...rceware.org, linux-api@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Deepa Dinamani <deepa.kernel@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Keith Packard <keithp@...thp.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC v2 PATCH] futex: extend set_robust_list to allow 2 locking
 ABIs at the same time.

On Tue, 5 Nov 2019, Florian Weimer wrote:
> * Thomas Gleixner:
> 
> > On Tue, 5 Nov 2019, Carlos O'Donell wrote:
> >> On 11/5/19 6:56 AM, Thomas Gleixner wrote:
> >> The other issue is this:
> >> 
> >> "Robust mutexes do not take ROBUST_LIST_LIMIT into account"
> >> https://sourceware.org/bugzilla/show_bug.cgi?id=19089
> >
> >   "The kernel limits the length of the robust mutex list to 2048 entries.
> >    This constant does not seem to be exported to user space."
> >
> > FWIW, the constant is defined in the UAPI futex header.
> >
> > The main concern here is not the actual number of futexes held by a task.
> >
> > The real issue is that the robust list could be circular by incident or
> > malice and there is no way for the kernel to figure that out. That would
> > prevent the task from exiting and make it iterate over the list until
> > doomsday, i.e. a nice unpriviledged DoS.
> >
> > So I fear the kernel cannot really help with this one.
> 
> I'm actually fine with treating ROBUST_LIST_LIMIT as an ABI constant.
> It's just not clear to me if the constant has this status today.  I
> suspect it was just split from the implementation headers at one point.

Yes, but we really can declare it as an ABI constant.

I think the limit is reasonably sized. But I'm not familiar with the lock
nesting expectations of insanely big enterprise applications.

Thanks,

	tglx