Re: [PATCH] mm: Add additional consistency check

[Michael Ellerman]

Kees Cook <keescook@xxxxxxxxxxxx> writes:

> On Fri, Mar 31, 2017 at 2:33 PM, Andrew Morton
> <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>> On Fri, 31 Mar 2017 09:40:28 -0700 Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>>
>>> As found in PaX, this adds a cheap check on heap consistency, just to
>>> notice if things have gotten corrupted in the page lookup.
>>
>> "As found in PaX" isn't a very illuminating justification for such a
>> change.  Was there a real kernel bug which this would have exposed, or
>> what?
>
> I don't know off the top of my head, but given the kinds of heap
> attacks I've been seeing, I think this added consistency check is
> worth it given how inexpensive it is. When heap metadata gets
> corrupted, we can get into nasty side-effects that can be
> attacker-controlled, so better to catch obviously bad states as early
> as possible.

There's your changelog :)

>>> --- a/mm/slab.h
>>> +++ b/mm/slab.h
>>> @@ -384,6 +384,7 @@ static inline struct kmem_cache *cache_from_obj(struct kmem_cache *s, void *x)
>>>               return s;
>>>
>>>       page = virt_to_head_page(x);
>>> +     BUG_ON(!PageSlab(page));
>>>       cachep = page->slab_cache;
>>>       if (slab_equal_or_root(cachep, s))
>>>               return cachep;
>>
>> BUG_ON might be too severe.  I expect the kindest VM_WARN_ON_ONCE()
>> would suffice here, but without more details it is hard to say.
>
> So, WARN isn't enough to protect the kernel (execution continues and
> the memory is still dereferenced for malicious purposes, etc).

You could do:

	if (WARN_ON(!PageSlab(page)))
        	return NULL.

Though I see at least two callers that don't check for a NULL return.

Looking at the context, the tail of the function already contains:

	pr_err("%s: Wrong slab cache. %s but object is from %s\n",
	       __func__, s->name, cachep->name);
	WARN_ON_ONCE(1);
	return s;
}

At least in slab.c it seems that would allow you to "free" an object
from one kmem_cache onto the array_cache of another kmem_cache, which
seems fishy. But maybe there's a check somewhere I'm missing?

cheers