Re: [PATCH v2 1/1] mm: fix use-after-free when anon vma name is used after vma is freed

From: Suren Baghdasaryan <surenb@google.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Colin Cross <ccross@google.com>,
	Sumit Semwal <sumit.semwal@linaro.org>,
	Michal Hocko <mhocko@suse.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Kees Cook <keescook@chromium.org>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Johannes Weiner <hannes@cmpxchg.org>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	brauner@kernel.org, legion@kernel.org, ran.xiaokai@zte.com.cn,
	sashal@kernel.org, Chris Hyser <chris.hyser@oracle.com>,
	Davidlohr Bueso <dave@stgolabs.net>,
	Peter Collingbourne <pcc@google.com>,
	caoxiaofeng@yulong.com, David Hildenbrand <david@redhat.com>,
	Cyrill Gorcunov <gorcunov@gmail.com>,
	linux-mm <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	kernel-team <kernel-team@android.com>,
	syzbot+aa7b3d4b35f9dc46a366@syzkaller.appspotmail.com
Subject: Re: [PATCH v2 1/1] mm: fix use-after-free when anon vma name is used after vma is freed
Date: Thu, 10 Feb 2022 07:21:52 -0800	[thread overview]
Message-ID: <CAJuCfpGhWh95wxRxy35d4jbCGUdCyasYQBs7DG2fVn34tSXrRQ@mail.gmail.com> (raw)
In-Reply-To: <YgUSxzp/qDwLkiKb@casper.infradead.org>

On Thu, Feb 10, 2022 at 5:27 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Wed, Feb 09, 2022 at 08:32:15PM -0800, Suren Baghdasaryan wrote:
> > +void vma_anon_name_put(struct anon_vma_name *anon_name)
> > +{
> > +     kref_put(&anon_name->kref, vma_anon_name_free);
> > +}
>
> To agree with Michal, make this:
>
>         if (anon_name)
>                 kref_put(&anon_name->kref, vma_anon_name_free);

Ack.

>
> >
> > -     error = madvise_update_vma(vma, prev, start, end, new_flags,
> > -                                vma_anon_name(vma));
> > +     anon_name = vma_anon_name_get(vma);
> > +     if (anon_name) {
> > +             error = madvise_update_vma(vma, prev, start, end, new_flags,
> > +                                        anon_name->name);
> > +             vma_anon_name_put(anon_name);
> > +     } else {
> > +             error = madvise_update_vma(vma, prev, start, end, new_flags,
> > +                                        NULL);
> > +     }
>
> And then this becomes:
>
>         anon_name = vma_anon_name_get(vma);
>         error = madvise_update_vma(vma, prev, start, end, new_flags, anon_name);
>         vma_anon_name_put(anon_name);

As I indicated in the other reply, there is another madvise_update_vma
user which has only the name string, not the anon_vma_name struct. So
this can become:

   anon_name = vma_anon_name_get(vma);
   error = madvise_update_vma(vma, prev, start, end, new_flags,
                                                   anon_name ?
anon_name->name : NULL);
   vma_anon_name_put(anon_name);