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

[ebiederm@xmission.com (Eric W. Biederman)]

ebiederm@xmission.com (Eric W. Biederman) writes:

> ebiederm@xmission.com (Eric W. Biederman) writes:
>
>> Peter Zijlstra <peterz@infradead.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.
>
> 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