linux-kernel - Re: [PATCH 41/41] mm: replace rw_semaphore with atomic_t in vma

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAJuCfpHkgRDpMJXd6vnCmB50PnSOwDY9pjhW5LcdR+fU5JUseA@mail.gmail.com>
Date:   Tue, 17 Jan 2023 10:36:42 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Jann Horn <jannh@...gle.com>, akpm@...ux-foundation.org,
        michel@...pinasse.org, jglisse@...gle.com, mhocko@...e.com,
        vbabka@...e.cz, hannes@...xchg.org, mgorman@...hsingularity.net,
        dave@...olabs.net, liam.howlett@...cle.com, peterz@...radead.org,
        ldufour@...ux.ibm.com, laurent.dufour@...ibm.com,
        paulmck@...nel.org, luto@...nel.org, songliubraving@...com,
        peterx@...hat.com, david@...hat.com, dhowells@...hat.com,
        hughd@...gle.com, bigeasy@...utronix.de, kent.overstreet@...ux.dev,
        punit.agrawal@...edance.com, lstoakes@...il.com,
        peterjung1337@...il.com, rientjes@...gle.com,
        axelrasmussen@...gle.com, joelaf@...gle.com, minchan@...gle.com,
        shakeelb@...gle.com, tatashin@...gle.com, edumazet@...gle.com,
        gthelen@...gle.com, gurua@...gle.com, arjunroy@...gle.com,
        soheil@...gle.com, hughlynch@...gle.com, leewalsh@...gle.com,
        posk@...gle.com, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH 41/41] mm: replace rw_semaphore with atomic_t in vma_lock

On Tue, Jan 17, 2023 at 10:31 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Tue, Jan 17, 2023 at 10:26:32AM -0800, Suren Baghdasaryan wrote:
> > On Tue, Jan 17, 2023 at 10:12 AM Jann Horn <jannh@...gle.com> wrote:
> > >
> > > On Mon, Jan 9, 2023 at 9:55 PM Suren Baghdasaryan <surenb@...gle.com> wrote:
> > > > rw_semaphore is a sizable structure of 40 bytes and consumes
> > > > considerable space for each vm_area_struct. However vma_lock has
> > > > two important specifics which can be used to replace rw_semaphore
> > > > with a simpler structure:
> > > [...]
> > > >  static inline void vma_read_unlock(struct vm_area_struct *vma)
> > > >  {
> > > > -       up_read(&vma->vm_lock->lock);
> > > > +       if (atomic_dec_and_test(&vma->vm_lock->count))
> > > > +               wake_up(&vma->vm_mm->vma_writer_wait);
> > > >  }
> > >
> > > I haven't properly reviewed this, but this bit looks like a
> > > use-after-free because you're accessing the vma after dropping your
> > > reference on it. You'd have to first look up the vma->vm_mm, then do
> > > the atomic_dec_and_test(), and afterwards do the wake_up() without
> > > touching the vma. Or alternatively wrap the whole thing in an RCU
> > > read-side critical section if the VMA is freed with RCU delay.
> >
> > vm_lock->count does not control the lifetime of the VMA, it's a
> > counter of how many readers took the lock or it's negative if the lock
> > is write-locked.
>
> Yes, but ...
>
>         Task A:
>         atomic_dec_and_test(&vma->vm_lock->count)
>                         Task B:
>                         munmap()
>                         write lock
>                         free VMA
>                         synchronize_rcu()
>                         VMA is really freed
>         wake_up(&vma->vm_mm->vma_writer_wait);
>
> ... vma is freed.
>
> Now, I think this doesn't occur.  I'm pretty sure that every caller of
> vma_read_unlock() is holding the RCU read lock.  But maybe we should
> have that assertion?

Yep, that's what this patch is doing
https://lore.kernel.org/all/20230109205336.3665937-27-surenb@google.com/
by calling vma_assert_no_reader() from __vm_area_free().

>