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Message-ID: <CAG48ez1z4F0sTBV3DCvkVJazQXxXGe=NyWKqYjKveEj-CQVy7w@mail.gmail.com>
Date: Mon, 24 Apr 2017 10:53:30 +0200
From: Jann Horn <jannh@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org,
Eric Biggers <ebiggers3@...il.com>, Christoph Hellwig <hch@...radead.org>,
"axboe@...nel.dk" <axboe@...nel.dk>, James Bottomley <James.Bottomley@...senpartnership.com>,
Elena Reshetova <elena.reshetova@...el.com>, Hans Liljestrand <ishkamiel@...il.com>,
David Windsor <dwindsor@...il.com>, x86@...nel.org, Ingo Molnar <mingo@...nel.org>,
Arnd Bergmann <arnd@...db.de>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jann Horn <jann@...jh.net>,
"David S. Miller" <davem@...emloft.net>, linux-arch@...r.kernel.org,
kernel-hardening@...ts.openwall.com, PaX Team <pageexec@...email.hu>
Subject: Re: Re: [PATCH] x86/refcount: Implement fast
refcount_t handling
On Mon, Apr 24, 2017 at 10:32 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, Apr 21, 2017 at 03:09:39PM -0700, Kees Cook wrote:
>> This patch ports the x86-specific atomic overflow handling from PaX's
>> PAX_REFCOUNT to the upstream refcount_t API. This is an updated version
>> from PaX that eliminates the saturation race condition by resetting the
>> atomic counter back to the INT_MAX saturation value on both overflow and
>> underflow. To win a race, a system would have to have INT_MAX threads
>> simultaneously overflow before the saturation handler runs.
>
> And is this impossible? Highly unlikely I'll grant you, but absolutely
> impossible?
>
> Also, you forgot nr_cpus in your bound. Afaict the worst case here is
> O(nr_tasks + 3*nr_cpus).
>
> Because PaX does it, is not a correctness argument. And this really
> wants one.
>From include/linux/threads.h:
/*
* A maximum of 4 million PIDs should be enough for a while.
* [NOTE: PID/TIDs are limited to 2^29 ~= 500+ million, see futex.h.]
*/
#define PID_MAX_LIMIT (CONFIG_BASE_SMALL ? PAGE_SIZE * 8 : \
(sizeof(long) > 4 ? 4 * 1024 * 1024 : PID_MAX_DEFAULT))
AFAICS that means you can only have up to 2^22 running tasks at
a time, since every running task requires a PID in the init pid namespace.
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