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

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Message-ID: <87a799nlsu.fsf@oldenburg2.str.redhat.com>
Date:   Wed, 06 Nov 2019 15:04:17 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     Zack Weinberg <zackw@...ix.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "Carlos O'Donell" <carlos@...hat.com>,
        Shawn Landden <shawn@....icu>,
        GNU C Library <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.

* Zack Weinberg:

> On Tue, Nov 5, 2019 at 9:28 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>>
>> 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.
>
> Why can't the kernel use the standard tortoise-and-hare algorithm for
> detecting circular linked lists here?

It's not guaranteed to terminate if the list is in shared memory.

Thanks,
Florian