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Date:   Tue, 17 Jan 2023 23:15:23 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     Jann Horn <jannh@google.com>, peterz@infradead.org,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
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Subject: Re: [PATCH 12/41] mm: add per-VMA lock and helper functions to
 control it
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On Tue, Jan 17, 2023 at 02:36:47PM -0800, Suren Baghdasaryan wrote:
> On Tue, Jan 17, 2023 at 1:46 PM Jann Horn <jannh@google.com> wrote:
> > On Tue, Jan 17, 2023 at 10:28 PM Suren Baghdasaryan <surenb@google.com> wrote:
> > > On Tue, Jan 17, 2023 at 10:03 AM Jann Horn <jannh@google.com> wrote:
> > > > One thing that might be gnarly here is that I think you might not be
> > > > allowed to use up_read() to fully release ownership of an object -
> > > > from what I remember, I think that up_read() (unlike something like
> > > > spin_unlock()) can access the lock object after it's already been
> > > > acquired by someone else. So if you want to protect against concurrent
> > > > deletion, this might have to be something like:
> > > >
> > > > rcu_read_lock(); /* keeps vma alive */
> > > > up_read(&vma->lock);
> > > > rcu_read_unlock();
> > >
> > > But for deleting VMA one would need to write-lock the vma->lock first,
> > > which I assume can't happen until this up_read() is complete. Is that
> > > assumption wrong?
> >
> > __up_read() does:
> >
> > rwsem_clear_reader_owned(sem);
> > tmp = atomic_long_add_return_release(-RWSEM_READER_BIAS, &sem->count);
> > DEBUG_RWSEMS_WARN_ON(tmp < 0, sem);
> > if (unlikely((tmp & (RWSEM_LOCK_MASK|RWSEM_FLAG_WAITERS)) ==
> >       RWSEM_FLAG_WAITERS)) {
> >   clear_nonspinnable(sem);
> >   rwsem_wake(sem);
> > }
> >
> > The atomic_long_add_return_release() is the point where we are doing
> > the main lock-releasing.
> >
> > So if a reader dropped the read-lock while someone else was waiting on
> > the lock (RWSEM_FLAG_WAITERS) and no other readers were holding the
> > lock together with it, the reader also does clear_nonspinnable() and
> > rwsem_wake() afterwards.
> > But in rwsem_down_write_slowpath(), after we've set
> > RWSEM_FLAG_WAITERS, we can return successfully immediately once
> > rwsem_try_write_lock() sees that there are no active readers or
> > writers anymore (if RWSEM_LOCK_MASK is unset and the cmpxchg
> > succeeds). We're not necessarily waiting for the "nonspinnable" bit or
> > the wake.
> >
> > So yeah, I think down_write() can return successfully before up_read()
> > is done with its memory accesses.
> >
> > (Spinlocks are different - the kernel relies on being able to drop
> > references via spin_unlock() in some places.)
> 
> Thanks for bringing this up. I can add rcu_read_{lock/unlock) as you
> suggested and that would fix the issue because we free VMAs from
> call_rcu(). However this feels to me as an issue of rw_semaphore
> design that this locking pattern is unsafe and might lead to UAF.

We have/had this problem with normal mutexes too.  It was the impetus
for adding the struct completion which is very careful to not touch
anything after the completion is, well, completed.