Re: [PATCH 2/2] mm: lock newly mapped VMA which can be modified after it becomes visible

[Suren Baghdasaryan]

On Fri, Jul 7, 2023 at 7:48 PM Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> wrote:
>
> * Suren Baghdasaryan <surenb@xxxxxxxxxx> [230707 00:32]:
> > mmap_region adds a newly created VMA into VMA tree and might modify it
> > afterwards before dropping the mmap_lock. This poses a problem for page
> > faults handled under per-VMA locks because they don't take the mmap_lock
> > and can stumble on this VMA while it's still being modified. Currently
> > this does not pose a problem since post-addition modifications are done
> > only for file-backed VMAs, which are not handled under per-VMA lock.
> > However, once support for handling file-backed page faults with per-VMA
> > locks is added, this will become a race.
> > Fix this by write-locking the VMA before inserting it into the VMA tree.
> > Other places where a new VMA is added into VMA tree do not modify it
> > after the insertion, so do not need the same locking.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > ---
> >  mm/mmap.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index c66e4622a557..84c71431a527 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -2812,6 +2812,8 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> >       if (vma->vm_file)
> >               i_mmap_lock_write(vma->vm_file->f_mapping);
> >
> > +     /* Lock the VMA since it is modified after insertion into VMA tree */
>
> So it is modified, but that i_mmap_lock_write() directly above this
> comment is potentially moving below the insert and that is why this lock
> is needed.

Correct, we should not rely on i_mmap_lock_write() which can be moved
(and is suggested to be moved in
https://lore.kernel.org/all/20230606124939.93561-1-yu.ma@xxxxxxxxx/).


>
> > +     vma_start_write(vma);
> >       vma_iter_store(&vmi, vma);
> >       mm->map_count++;
> >       if (vma->vm_file) {
> > --
> > 2.41.0.255.g8b1d071c50-goog
> >
>
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