linux-kernel - Re: [PATCH 0/3] ipc subsystem refcounter conversions

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Date:   Mon, 29 May 2017 12:24:42 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
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@...nel.dk" <axboe@...nel.dk>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        "x86@...nel.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@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] ipc subsystem refcounter conversions

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'.

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.