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    SubjectRe: [PATCH 12/41] mm: add per-VMA lock and helper functions to control it
    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@google.com> 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.
    --
    Michal Hocko
    SUSE Labs

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