Re: [PATCH 02/21] binder: fix use-after-free in shinker's callback

[Carlos Llamas <cmllamas@google.com>]

On Thu, Nov 02, 2023 at 03:20:51PM -0400, Liam R. Howlett wrote:
> * Carlos Llamas <cmllamas@google.com> [231102 15:00]:
> > The mmap read lock is used during the shrinker's callback, which means
> > that using alloc->vma pointer isn't safe as it can race with munmap().
> 
> I think you know my feelings about the safety of that pointer from
> previous discussions.
> 

Yeah. The work here is not done. We actually already store the vm_start
address in alloc->buffer, so in theory we don't even need to swap the
alloc->vma pointer we could just drop it. So, I agree with you.

I want to include this saftey "fix" along with some other work that uses
the page fault handler and get_user_pages_remote(). I've tried a quick
prototype of this and it works fine.

> > diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
> > index e3db8297095a..c4d60d81221b 100644
> > --- a/drivers/android/binder_alloc.c
> > +++ b/drivers/android/binder_alloc.c
> > @@ -1005,7 +1005,9 @@ enum lru_status binder_alloc_free_page(struct list_head *item,
> >  		goto err_mmget;
> >  	if (!mmap_read_trylock(mm))
> >  		goto err_mmap_read_lock_failed;
> > -	vma = binder_alloc_get_vma(alloc);
> > +	vma = vma_lookup(mm, page_addr);
> > +	if (vma && vma != binder_alloc_get_vma(alloc))
> > +		goto err_invalid_vma;
> 
> Doesn't this need to be:
> if (!vma || vma != binder_alloc_get_vma(alloc))
> 
> This way, we catch a different vma and a NULL vma.
> 
> Or even, just:
> if (vma != binder_alloc_get_vma(alloc))
> 
> if the alloc vma cannot be NULL?
> 

If the vma_lookup() is NULL then we still need to isolate and free the
given binder page and we obviously skip the zap() in this case.

However, if we receive a random unexpected vma because of a corrupted
address or similar, then the whole process is skipped.

Thus, why we use the check above.

--
Carlos Llamas