Re: [PATCH 12/41] mm: add per-VMA lock and helper functions to control it

[Jann Horn]

On Wed, Jan 18, 2023 at 1:28 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> On Tue 17-01-23 19:02:55, Jann Horn wrote:
> > +locking maintainers
> >
> > On Mon, Jan 9, 2023 at 9:54 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> > > Introduce a per-VMA rw_semaphore to be used during page fault handling
> > > instead of mmap_lock. Because there are cases when multiple VMAs need
> > > to be exclusively locked during VMA tree modifications, instead of the
> > > usual lock/unlock patter we mark a VMA as locked by taking per-VMA lock
> > > exclusively and setting vma->lock_seq to the current mm->lock_seq. When
> > > mmap_write_lock holder is done with all modifications and drops mmap_lock,
> > > it will increment mm->lock_seq, effectively unlocking all VMAs marked as
> > > locked.
> > [...]
> > > +static inline void vma_read_unlock(struct vm_area_struct *vma)
> > > +{
> > > +       up_read(&vma->lock);
> > > +}
> >
> > 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.
>
> Yes, I think you are right. From a look into the code it seems that
> the UAF is quite unlikely as there is a ton of work to be done between
> vma_write_lock used to prepare vma for removal and actual removal.
> That doesn't make it less of a problem though.
>
> > 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 I'm not entirely sure about that, the locking folks might know better.
>
> I am not a locking expert but to me it looks like this should work
> because the final cleanup would have to happen rcu_read_unlock.
>
> Thanks, I have completely missed this aspect of the locking when looking
> into the code.
>
> Btw. looking at this again I have fully realized how hard it is actually
> to see that vm_area_free is guaranteed to sync up with ongoing readers.
> vma manipulation functions like __adjust_vma make my head spin. Would it
> make more sense to have a rcu style synchronization point in
> vm_area_free directly before call_rcu? This would add an overhead of
> uncontended down_write of course.

Something along those lines might be a good idea, but I think that
rather than synchronizing the removal, it should maybe be something
that splats (and bails out?) if it detects pending readers. If we get
to vm_area_free() on a VMA that has pending readers, we might already
be in a lot of trouble because the concurrent readers might have been
traversing page tables while we were tearing them down or fun stuff
like that.

I think maybe Suren was already talking about something like that in
another part of this patch series but I don't remember...