Re: Is it OK to pass non-acquired objects to kfree?

[Dmitry Vyukov <dvyukov@google.com>]

On Tue, Sep 8, 2015 at 7:09 PM, Christoph Lameter <cl@linux.com> wrote:
> On Tue, 8 Sep 2015, Dmitry Vyukov wrote:
>
>> >> I would expect that this is illegal code. Is my understanding correct?
>> >
>> > This should work. It could be a problem if thread 1 is touching
>> > the object.
>>
>> What does make it work?
>
> The 2nd thread gets the pointer that the first allocated and frees it.
> If there is no more processing then fine.
>
>> There are clearly memory barriers missing when passing the object
>> between threads. The typical correct pattern is:
>
> Why? If thread 2 gets the pointer it frees it. Thats ok.
>
>> // thread 1
>> smp_store_release(&p, kmalloc(8));
>>
>> // thread 2
>> void *r = smp_load_acquire(&p); // or READ_ONCE_CTRL
>> if (r)
>>   kfree(r);
>>
>> Otherwise stores into the object in kmalloc can reach the object when
>> it is already freed, which is a use-after-free.
>
> Ok so there is more code executing in thread #1. That changes things.
>>
>> What does prevent the use-after-free?
>
> There is no access to p in the first thread. If there are such accesses
> then they are illegal. A user of slab allocators must ensure that there
> are no accesses after freeing the object. And since there is a thread
> that  at random checks p and frees it when not NULL then no other thread
> would be allowed to touch the object.


But the memory allocator itself (kmalloc/kfree) generally reads and
writes the object (e.g. storing object size in header before object,
writing redzone in debug mode, reading and checking redzone in debug
mode, building freelist using first word of the object, etc). There is
no different between user accesses and memory allocator accesses just
before returning the object from kmalloc and right after accepting the
object in kfree.


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Dmitry Vyukov, Software Engineer, dvyukov@google.com
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