Re: [RFC]: Possible race condition in kernel futex code

From	Hans Zuidam <h.zuidam@online.nl>
Newsgroups	linux.kernel
Subject	Re: [RFC]: Possible race condition in kernel futex code
Date	2015-10-09 12:00 +0200
Message-ID	<qhCCZ-6Yo-3@gated-at.bofh.it> (permalink)
References	<qgfSa-2Pr-33@gated-at.bofh.it> <qhBQC-63L-23@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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Hi Thomas,

On 9 okt. 2015, at 11:06, Thomas Gleixner <tglx@linutronix.de> wrote:
On Mon, 5 Oct 2015, Jaccon Bastiaansen wrote:
>> We did some tests with different compilers, kernel versions and kernel
>> configs, with the following results:

> You cannot use an explicit 32bit read. We need an access which handles the fault gracefully.

The reason for the explicit read suggestion is to avoid the _builtin_constant_p() in __copy_from_user_nocheck().  The GCC manual says that there may be situations where it returns 0 even though the argument is a constant.  Although none of the compiler/kernel combinations we have tried showed this happening, we think it is probably better to be safe than sorry.

With kind regards,
Hans Zuidam

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Thread

Re: [RFC]: Possible race condition in kernel futex code Thomas Gleixner <tglx@linutronix.de> - 2015-10-09 11:10 +0200
  Re: [RFC]: Possible race condition in kernel futex code Hans Zuidam <h.zuidam@online.nl> - 2015-10-09 12:00 +0200
    Re: [RFC]: Possible race condition in kernel futex code Thomas Gleixner <tglx@linutronix.de> - 2015-10-09 12:30 +0200
      Re: [RFC]: Possible race condition in kernel futex code Peter Zijlstra <peterz@infradead.org> - 2015-10-09 13:40 +0200
  Re: [RFC]: Possible race condition in kernel futex code Greg KH <greg@kroah.com> - 2015-10-18 02:30 +0200

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