RE: [PATCH v2 1/2] mm, kasan: improve double-free detection

["Luruo, Kuthonuzo" <kuthonuzo.luruo@hpe.com>]

Thank you for the review!

> > +
> > +/* acquire per-object lock for access to KASAN metadata. */
> 
> I believe there's strong reason not to use standard spin_lock() or
> similar. I think it's proper place to explain it.
> 

will do.

> > +void kasan_meta_lock(struct kasan_alloc_meta *alloc_info)
> > +{
> > +	union kasan_alloc_data old, new;
> > +
> > +	preempt_disable();
> 
> It's better to disable and enable preemption inside the loop
> on each iteration, to decrease contention.
> 

ok, makes sense; will do.

> > +	for (;;) {
> > +		old.packed = READ_ONCE(alloc_info->data);
> > +		if (unlikely(old.lock)) {
> > +			cpu_relax();
> > +			continue;
> > +		}
> > +		new.packed = old.packed;
> > +		new.lock = 1;
> > +		if (likely(cmpxchg(&alloc_info->data, old.packed, new.packed)
> > +					== old.packed))
> > +			break;
> > +	}
> > +}
> > +
> > +/* release lock after a kasan_meta_lock(). */
> > +void kasan_meta_unlock(struct kasan_alloc_meta *alloc_info)
> > +{
> > +	union kasan_alloc_data alloc_data;
> > +
> > +	alloc_data.packed = READ_ONCE(alloc_info->data);
> > +	alloc_data.lock = 0;
> > +	if (unlikely(xchg(&alloc_info->data, alloc_data.packed) !=
> > +				(alloc_data.packed | 0x1U)))
> > +		WARN_ONCE(1, "%s: lock not held!\n", __func__);
> 
> Nitpick. It never happens in normal case, correct?. Why don't you place it under
> some developer config, or even leave at dev branch? The function will
> be twice shorter without it.

ok, will remove/shorten.

> > +	alloc_data.packed = alloc_info->data;
> > +	if (alloc_data.state == KASAN_STATE_ALLOC) {
> > +		free_info = get_free_info(cache, object);
> > +		quarantine_put(free_info, cache);
> 
> I just pulled master and didn't find this function. If your patchset
> is based on other branch, please notice it.

Sorry; patchset is based on linux-next 'next-20160506' which has Alexander
Potapenko's patches for KASAN SLAB support with memory quarantine +
stackdepot features.

> 
> > +		set_track(&free_info->track, GFP_NOWAIT);
> 
> It may fail for many reasons. Is it OK to ignore it? If OK, I think it
> should be explained.

It's ok. A subsequent bug report on object would have a missing alloc/dealloc
stack trace. 

> 
> > +		kasan_poison_slab_free(cache, object);
> > +		alloc_data.state = KASAN_STATE_QUARANTINE;
> > +		alloc_info->data = alloc_data.packed;
> > +		kasan_meta_unlock(alloc_info);
> > +		return true;
> >  	}
> > +	switch (alloc_data.state) {
> > +	case KASAN_STATE_QUARANTINE:
> > +	case KASAN_STATE_FREE:
> > +		kasan_report((unsigned long)object, 0, false,
> > +				(unsigned long)__builtin_return_address(1));
> 
> __builtin_return_address() is unsafe if argument is non-zero. Use
> return_address() instead.

hmm, I/cscope can't seem to find an x86 implementation for return_address().
Will dig further; thanks.

> > +		local_irq_save(flags);
> > +		kasan_meta_lock(alloc_info);
> > +		alloc_data.packed = alloc_info->data;
> > +		alloc_data.state = KASAN_STATE_ALLOC;
> > +		alloc_data.size_delta = cache->object_size - size;
> > +		alloc_info->data = alloc_data.packed;
> >  		set_track(&alloc_info->track, flags);
> 
> Same as above
>
As above. 

Kuthonuzo

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