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

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Date:   Thu, 2 Nov 2023 16:27:00 -0400
From:   "Liam R. Howlett" <Liam.Howlett@...cle.com>
To:     Carlos Llamas <cmllamas@...gle.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Christian Brauner <brauner@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...nel.org>, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com, stable@...r.kernel.org,
        Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH 02/21] binder: fix use-after-free in shinker's callback

* Carlos Llamas <cmllamas@...gle.com> [231102 16:09]:

...
> 
> > > 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.

I would have thought if there was no VMA, then the entire process could
be avoided.  Thanks for clarifying.

> 
> 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