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 15:27:54 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Carlos O'Donell <carlos@...hat.com>
cc:     Florian Weimer <fweimer@...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, 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.

Thanks,

	tglx