From mboxrd@z Thu Jan 1 00:00:00 1970 From: ebiederm@xmission.com (Eric W. Biederman) Subject: Re: [PATCH 0/3] ipc subsystem refcounter conversions Date: Mon, 29 May 2017 06:39:44 -0500 Message-ID: <8760gjvqm7.fsf@xmission.com> References: <1487590189-18151-1-git-send-email-elena.reshetova@intel.com> <20170303162352.b6af1c0c3115b3f5f1e7aed3@linux-foundation.org> <20170529083903.GA17735@infradead.org> <87h904xc26.fsf@xmission.com> <20170529102442.gerbzxzixllen46q@hirez.programming.kicks-ass.net> <87a85wvsxa.fsf@xmission.com> <87o9ubvr0t.fsf@xmission.com> Mime-Version: 1.0 Content-Type: text/plain Return-path: Received: from out02.mta.xmission.com ([166.70.13.232]:51193 "EHLO out02.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750871AbdE2Lqd (ORCPT ); Mon, 29 May 2017 07:46:33 -0400 In-Reply-To: <87o9ubvr0t.fsf@xmission.com> (Eric W. Biederman's message of "Mon, 29 May 2017 06:30:58 -0500") Sender: linux-arch-owner@vger.kernel.org List-ID: To: Peter Zijlstra Cc: Christoph Hellwig , Kees Cook , Andrew Morton , Elena Reshetova , Greg KH , Ingo Molnar , Alexey Dobriyan , "Serge E. Hallyn" , arozansk@redhat.com, Davidlohr Bueso , Manfred Spraul , "axboe@kernel.dk" , James Bottomley , "x86@kernel.org" , Ingo Molnar , Arnd Bergmann , "David S. Miller" , Rik van Riel , linux-arch , kernel-hardening@lists.openwall.com ebiederm@xmission.com (Eric W. Biederman) writes: > ebiederm@xmission.com (Eric W. Biederman) writes: > >> Peter Zijlstra writes: >> >>> On Mon, May 29, 2017 at 04:11:13AM -0500, Eric W. Biederman wrote: >>> >>>> Kees I I have a concern: >>>> >>>> __must_check bool refcount_add_not_zero(unsigned int i, refcount_t *r) >>>> { >>>> unsigned int new, val = atomic_read(&r->refs); >>>> >>>> do { >>>> if (!val) >>>> return false; >>>> >>>> if (unlikely(val == UINT_MAX)) >>>> return true; >>>> >>>> new = val + i; >>>> if (new < val) >>>> new = UINT_MAX; >>>> >>>> } while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new)); >>>> >>>> WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n"); >>>> >>>> return true; >>>> } >>>> >>>> Why in the world do you succeed when you the value saturates???? >>> >>> Why not? On saturation the object will leak and returning a reference to >>> it is always good. >>> >>>> From a code perspective that is bizarre. The code already has to handle >>>> the case when the counter does not increment. >>> >>> I don't see it as bizarre, we turned an overflow/use-after-free into a >>> leak. That's the primary mechanism here. >>> >>> As long as we have a reference to a leaked object, we might as well use >>> it, its not going anywhere. >>> >>>> Fixing the return value would move refcount_t into the realm of >>>> something that is desirable because it has bettern semantics and >>>> is more useful just on a day to day correctness point of view. Even >>>> ignoring the security implications. >>> >>> It changes the semantics between inc_not_zero() and inc(). It also >>> complicates the semantics of inc_not_zero(), where currently the failure >>> implies the count is 0 and means no-such-object, you complicate matters >>> by basically returning 'busy'. >> >> Busy is not a state of a reference count. >> >> It is true I am suggesting treating something with a saturated reference >> as not available. If that is what you mean by busy. But if it's >> reference is zero it is also not available. So there is no practical >> difference. >> >>> That is a completely new class of failure that is actually hard to deal >>> with, not to mention that it completely destroys refcount_inc_not_zero() >>> being a 'simple' replacement for atomic_inc_not_zero(). >>> >>> In case of the current failure, the no-such-object, we can fix that by >>> creating said object. But what to do on 'busy' ? Surely you don't want >>> to create another. You'd have to somehow retrofit something to wait on >>> in every user. >> >> Using little words. >> >> A return of true from inc_not_zero means we took a reference. >> A return of false means we did not take a reference. >> >> The code already handles I took a reference or I did not take a >> reference. >> >> Therefore lying with refcount_t is not helpful. It takes failures >> the code could easily handle and turns them into leaks. >> >> At least that is how I have seen reference counts used. And those >> are definitely the plane obivous semantics. >> >> Your changes are definitely not drop in replacements for atomic_t in my >> code. > > To clarify. > > If my code uses atomic_inc it does not expect a failure of any sort > and saturate semantics are a fine replacement. > > If my code uses atomic_inc_not_zero it knows how to handle a failure > to take a reference count. Making hiding the failure really bizarre. > > A must check function that hides a case I can handle and requires > checking in a case where my code is built not to check is a drop in > replacement for neither. > > So anyone who is proposing a refcount_t change as a drop in replacement > for any code I maintain I will nack on sight because refcount_t is not > currently a no-brain drop in replacement. *Blink* I failed to see that there is a refcount_inc. Too much noise in the header file I suppose. But implementing refcount_inc in terms of refcount_inc_not_zero is totally broken. The two operations are not the same and the go to different assumptions the code is making. That explains why you think refcount_inc_not_zero should lie because you are implementing refcount_inc with it. They are semantically very different operations. Please separate them. Eric From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from out02.mta.xmission.com ([166.70.13.232]:51193 "EHLO out02.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750871AbdE2Lqd (ORCPT ); Mon, 29 May 2017 07:46:33 -0400 From: ebiederm@xmission.com (Eric W. Biederman) References: <1487590189-18151-1-git-send-email-elena.reshetova@intel.com> <20170303162352.b6af1c0c3115b3f5f1e7aed3@linux-foundation.org> <20170529083903.GA17735@infradead.org> <87h904xc26.fsf@xmission.com> <20170529102442.gerbzxzixllen46q@hirez.programming.kicks-ass.net> <87a85wvsxa.fsf@xmission.com> <87o9ubvr0t.fsf@xmission.com> Date: Mon, 29 May 2017 06:39:44 -0500 In-Reply-To: <87o9ubvr0t.fsf@xmission.com> (Eric W. Biederman's message of "Mon, 29 May 2017 06:30:58 -0500") Message-ID: <8760gjvqm7.fsf@xmission.com> MIME-Version: 1.0 Content-Type: text/plain Subject: Re: [PATCH 0/3] ipc subsystem refcounter conversions Sender: linux-arch-owner@vger.kernel.org List-ID: To: Peter Zijlstra Cc: Christoph Hellwig , Kees Cook , Andrew Morton , Elena Reshetova , Greg KH , Ingo Molnar , Alexey Dobriyan , "Serge E. Hallyn" , arozansk@redhat.com, Davidlohr Bueso , Manfred Spraul , "axboe@kernel.dk" , James Bottomley , "x86@kernel.org" , Ingo Molnar , Arnd Bergmann , "David S. Miller" , Rik van Riel , linux-arch , "kernel-hardening@lists.openwall.com" , LKML Message-ID: <20170529113944.-D1xuz6-VjhkcmQbM8uQtskWDnx-qOdRZhljbUbkEbs@z> ebiederm@xmission.com (Eric W. Biederman) writes: > ebiederm@xmission.com (Eric W. Biederman) writes: > >> Peter Zijlstra writes: >> >>> On Mon, May 29, 2017 at 04:11:13AM -0500, Eric W. Biederman wrote: >>> >>>> Kees I I have a concern: >>>> >>>> __must_check bool refcount_add_not_zero(unsigned int i, refcount_t *r) >>>> { >>>> unsigned int new, val = atomic_read(&r->refs); >>>> >>>> do { >>>> if (!val) >>>> return false; >>>> >>>> if (unlikely(val == UINT_MAX)) >>>> return true; >>>> >>>> new = val + i; >>>> if (new < val) >>>> new = UINT_MAX; >>>> >>>> } while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new)); >>>> >>>> WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n"); >>>> >>>> return true; >>>> } >>>> >>>> Why in the world do you succeed when you the value saturates???? >>> >>> Why not? On saturation the object will leak and returning a reference to >>> it is always good. >>> >>>> From a code perspective that is bizarre. The code already has to handle >>>> the case when the counter does not increment. >>> >>> I don't see it as bizarre, we turned an overflow/use-after-free into a >>> leak. That's the primary mechanism here. >>> >>> As long as we have a reference to a leaked object, we might as well use >>> it, its not going anywhere. >>> >>>> Fixing the return value would move refcount_t into the realm of >>>> something that is desirable because it has bettern semantics and >>>> is more useful just on a day to day correctness point of view. Even >>>> ignoring the security implications. >>> >>> It changes the semantics between inc_not_zero() and inc(). It also >>> complicates the semantics of inc_not_zero(), where currently the failure >>> implies the count is 0 and means no-such-object, you complicate matters >>> by basically returning 'busy'. >> >> Busy is not a state of a reference count. >> >> It is true I am suggesting treating something with a saturated reference >> as not available. If that is what you mean by busy. But if it's >> reference is zero it is also not available. So there is no practical >> difference. >> >>> That is a completely new class of failure that is actually hard to deal >>> with, not to mention that it completely destroys refcount_inc_not_zero() >>> being a 'simple' replacement for atomic_inc_not_zero(). >>> >>> In case of the current failure, the no-such-object, we can fix that by >>> creating said object. But what to do on 'busy' ? Surely you don't want >>> to create another. You'd have to somehow retrofit something to wait on >>> in every user. >> >> Using little words. >> >> A return of true from inc_not_zero means we took a reference. >> A return of false means we did not take a reference. >> >> The code already handles I took a reference or I did not take a >> reference. >> >> Therefore lying with refcount_t is not helpful. It takes failures >> the code could easily handle and turns them into leaks. >> >> At least that is how I have seen reference counts used. And those >> are definitely the plane obivous semantics. >> >> Your changes are definitely not drop in replacements for atomic_t in my >> code. > > To clarify. > > If my code uses atomic_inc it does not expect a failure of any sort > and saturate semantics are a fine replacement. > > If my code uses atomic_inc_not_zero it knows how to handle a failure > to take a reference count. Making hiding the failure really bizarre. > > A must check function that hides a case I can handle and requires > checking in a case where my code is built not to check is a drop in > replacement for neither. > > So anyone who is proposing a refcount_t change as a drop in replacement > for any code I maintain I will nack on sight because refcount_t is not > currently a no-brain drop in replacement. *Blink* I failed to see that there is a refcount_inc. Too much noise in the header file I suppose. But implementing refcount_inc in terms of refcount_inc_not_zero is totally broken. The two operations are not the same and the go to different assumptions the code is making. That explains why you think refcount_inc_not_zero should lie because you are implementing refcount_inc with it. They are semantically very different operations. Please separate them. Eric