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

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Message-ID: <CAJuCfpHPZNbK2rd95pZAWr0d3vACGzHdiAFv=bM-G8nC+YdYwg@mail.gmail.com>
Date:   Thu, 10 Feb 2022 08:00:15 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Colin Cross <ccross@...gle.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Kees Cook <keescook@...omium.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Johannes Weiner <hannes@...xchg.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>, brauner@...nel.org,
        legion@...nel.org, ran.xiaokai@....com.cn, sashal@...nel.org,
        Chris Hyser <chris.hyser@...cle.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Peter Collingbourne <pcc@...gle.com>, caoxiaofeng@...ong.com,
        David Hildenbrand <david@...hat.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>,
        syzbot+aa7b3d4b35f9dc46a366@...kaller.appspotmail.com
Subject: Re: [PATCH v2 1/1] mm: fix use-after-free when anon vma name is used
 after vma is freed

On Thu, Feb 10, 2022 at 7:27 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Thu, Feb 10, 2022 at 07:18:24AM -0800, Suren Baghdasaryan wrote:
> > On Thu, Feb 10, 2022 at 4:40 AM 'Michal Hocko' via kernel-team
> > <kernel-team@...roid.com> wrote:
> > >
> > > On Wed 09-02-22 20:32:15, Suren Baghdasaryan wrote:
> > > > When adjacent vmas are being merged it can result in the vma that was
> > > > originally passed to madvise_update_vma being destroyed. In the current
> > > > implementation, the name parameter passed to madvise_update_vma points
> > > > directly to vma->anon_name->name and it is used after the call to
> > > > vma_merge. In the cases when vma_merge merges the original vma and
> > > > destroys it, this will result in use-after-free bug as shown below:
> > > >
> > > > madvise_vma_behavior << passes vma->anon_name->name as name param
> > > >   madvise_update_vma(name)
> > > >     vma_merge
> > > >       __vma_adjust
> > > >         vm_area_free <-- frees the vma
> > > >     replace_vma_anon_name(name) <-- UAF
> > > >
> > > > Fix this by raising the name refcount and stabilizing it. Introduce
> > > > vma_anon_name_{get/put} API for this purpose.
> > >
> > > What is the reason that madvise_update_vma uses the naked name rather
> > > than the encapsulated anon_vma_name? This really just begs for problems.
> >
> > The reason for that is the second place it's being used from the prctl syscall:
> >
> > prctl_set_vma
> >   madvise_set_anon_name
> >     madvise_vma_anon_name
> >       madvise_update_vma
> >
> > In that case the name parameter is not part of any anon_vma_name
> > struct and therefore is stable. I can add a comment to
> > madvise_update_vma indicating that the name parameter has to be stable
> > if that helps.
>
> Seems to me it'd simplify things if replace_vma_anon_name() and
> madvise_vma_anon_name() took a struct anon_vma_name instead of
> a bare char *.  You could construct it in madvise_set_anon_name().

Ok, this can be done. However I don't think changing
replace_vma_anon_name() to accept a struct anon_vma_name would be a
good idea. Reader might think that the object being passed will become
the vma->anon_name of the vma, while in reality that's not the case.
Keeping it a char* makes it obvious that the function will construct a
new anon_vma_name struct.
I'll post a v3 shortly implementing these suggestions.
Thanks for the review folks!