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Date: Mon, 29 May 2017 05:49:53 -0500
From: ebiederm@...ssion.com (Eric W. Biederman)
To: Peter Zijlstra <peterz@...radead.org>
Cc: Christoph Hellwig <hch@...radead.org>, Kees Cook <keescook@...omium.org>, Andrew Morton <akpm@...ux-foundation.org>, Elena Reshetova <elena.reshetova@...el.com>, Greg KH <gregkh@...uxfoundation.org>, Ingo Molnar <mingo@...hat.com>, Alexey Dobriyan <adobriyan@...il.com>, "Serge E. Hallyn" <serge@...lyn.com>, arozansk@...hat.com, Davidlohr Bueso <dave@...olabs.net>, Manfred Spraul <manfred@...orfullife.com>, "axboe\@kernel.dk" <axboe@...nel.dk>, James Bottomley <James.Bottomley@...senpartnership.com>, "x86\@kernel.org" <x86@...nel.org>, Ingo Molnar <mingo@...nel.org>, Arnd Bergmann <arnd@...db.de>, "David S. Miller" <davem@...emloft.net>, Rik van Riel <riel@...hat.com>, linux-arch <linux-arch@...r.kernel.org>, "kernel-hardening\@lists.openwall.com" <kernel-hardening@...ts.openwall.com>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] ipc subsystem refcounter conversions
Peter Zijlstra <peterz@...radead.org> 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.
Eric
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