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SubjectRe: More filesystem need this fix (xfs: use MMAPLOCK around filemap_map_pages())
On Thu, Sep 17, 2020 at 11:44:54AM +1000, Dave Chinner wrote:
> So....
>
> P0 p1
>
> hole punch starts
> takes XFS_MMAPLOCK_EXCL
> truncate_pagecache_range()

... locks page ...

> unmap_mapping_range(start, end)
> <clears ptes>
> <read fault>
> do_fault_around()
> ->map_pages
> filemap_map_pages()

... trylock page fails ...

> page mapping valid,
> page is up to date
> maps PTEs
> <fault done>
> truncate_inode_pages_range()
> truncate_cleanup_page(page)
> invalidates page
> delete_from_page_cache_batch(page)
> frees page
> <pte now points to a freed page>
>
> That doesn't seem good to me.
>
> Sure, maybe the page hasn't been freed back to the free lists
> because of elevated refcounts. But it's been released by the
> filesystem and not longer in the page cache so nothing good can come
> of this situation...
>
> AFAICT, this race condition exists for the truncate case as well
> as filemap_map_pages() doesn't have a "page beyond inode size" check
> in it. However, exposure here is very limited in the truncate case
> because truncate_setsize()->truncate_pagecache() zaps the PTEs
> again after invalidating the page cache.
>
> Either way, adding the XFS_MMAPLOCK_SHARED around
> filemap_map_pages() avoids the race condition for fallocate and
> truncate operations for XFS...
>
> > As such it is a rather
> > different beast compared to the fault handler from fs POV and does not need
> > protection from hole punching (current serialization on page lock and
> > checking of page->mapping is enough).
> > That being said I agree this is subtle and the moment someone adds e.g. a
> > readahead call into filemap_map_pages() we have a real problem. I'm not
> > sure how to prevent this risk...
>
> Subtle, yes. So subtle, in fact, I fail to see any reason why the
> above race cannot occur as there's no obvious serialisation in the
> page cache between PTE zapping and page invalidation to prevent a
> fault from coming in an re-establishing the PTEs before invalidation
> occurs.
>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@fromorbit.com
>

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