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Re: [kernel-hardening] Re: [PATCH] x86/refcount: Implement fast refcount_t handling

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On Mon, Apr 24, 2017 at 10:32 AM, Peter Zijlstra <peterz@infradead.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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[PATCH] x86/refcount: Implement fast refcount_t handling Kees Cook <keescook@chromium.org> - 2017-04-22 00:10 +0200
  Re: [PATCH] x86/refcount: Implement fast refcount_t handling Peter Zijlstra <peterz@infradead.org> - 2017-04-24 10:40 +0200
    Re: [kernel-hardening] Re: [PATCH] x86/refcount: Implement fast  refcount_t handling Jann Horn <jannh@google.com> - 2017-04-24 11:00 +0200
      Re: [kernel-hardening] Re: [PATCH] x86/refcount: Implement fast  refcount_t handling Peter Zijlstra <peterz@infradead.org> - 2017-04-24 11:30 +0200
    Re: [PATCH] x86/refcount: Implement fast refcount_t handling "PaX Team" <pageexec@freemail.hu> - 2017-04-24 13:10 +0200
      Re: [PATCH] x86/refcount: Implement fast refcount_t handling Peter Zijlstra <peterz@infradead.org> - 2017-04-24 13:20 +0200
        Re: [PATCH] x86/refcount: Implement fast refcount_t handling "PaX Team" <pageexec@freemail.hu> - 2017-04-24 15:20 +0200
          Re: [PATCH] x86/refcount: Implement fast refcount_t handling Peter Zijlstra <peterz@infradead.org> - 2017-04-24 15:40 +0200
            Re: [PATCH] x86/refcount: Implement fast refcount_t handling "PaX Team" <pageexec@freemail.hu> - 2017-04-24 17:20 +0200
              Re: [PATCH] x86/refcount: Implement fast refcount_t handling Kees Cook <keescook@chromium.org> - 2017-04-24 22:50 +0200
      Re: [PATCH] x86/refcount: Implement fast refcount_t handling Kees Cook <keescook@chromium.org> - 2017-04-24 22:40 +0200
    Re: [PATCH] x86/refcount: Implement fast refcount_t handling Kees Cook <keescook@chromium.org> - 2017-04-24 22:20 +0200
  Re: [PATCH] x86/refcount: Implement fast refcount_t handling Peter Zijlstra <peterz@infradead.org> - 2017-04-24 12:50 +0200
    Re: [PATCH] x86/refcount: Implement fast refcount_t handling Kees Cook <keescook@chromium.org> - 2017-04-24 22:20 +0200
  Re: [PATCH] x86/refcount: Implement fast refcount_t handling Peter Zijlstra <peterz@infradead.org> - 2017-04-24 13:00 +0200
    Re: [PATCH] x86/refcount: Implement fast refcount_t handling Kees Cook <keescook@chromium.org> - 2017-04-24 22:30 +0200

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