Re: [PATCH mm-unstable] mm/khugepaged: fix collapse_pte_mapped_thp() versus uffd

[Peter Xu <peterx@redhat.com>]

On Mon, Aug 21, 2023 at 12:51:20PM -0700, Hugh Dickins wrote:
> Jann Horn demonstrated how userfaultfd ioctl UFFDIO_COPY into a private
> shmem mapping can add valid PTEs to page table collapse_pte_mapped_thp()
> thought it had emptied: page lock on the huge page is enough to protect
> against WP faults (which find the PTE has been cleared), but not enough
> to protect against userfaultfd.  "BUG: Bad rss-counter state" followed.
> 
> retract_page_tables() protects against this by checking !vma->anon_vma;
> but we know that MADV_COLLAPSE needs to be able to work on private shmem
> mappings, even those with an anon_vma prepared for another part of the
> mapping; and we know that MADV_COLLAPSE needs to work on shared shmem
> mappings which are userfaultfd_armed().  Whether it needs to work on
> private shmem mappings which are userfaultfd_armed(), I'm not so sure:
> but assume that it does.
> 
> Just for this case, take the pmd_lock() two steps earlier: not because
> it gives any protection against this case itself, but because ptlock
> nests inside it, and it's the dropping of ptlock which let the bug in.
> In other cases, continue to minimize the pmd_lock() hold time.
> 
> Reported-by: Jann Horn <jannh@google.com>
> Closes: https://lore.kernel.org/linux-mm/CAG48ez0FxiRC4d3VTu_a9h=rg5FW-kYD5Rg5xo_RDBM0LTTqZQ@mail.gmail.com/
> Fixes: 1043173eb5eb ("mm/khugepaged: collapse_pte_mapped_thp() with mmap_read_lock()")
> Signed-off-by: Hugh Dickins <hughd@google.com>

The locking is indeed slightly complicated.. but I didn't spot anything
wrong.

Acked-by: Peter Xu <peterx@redhat.com>

Thanks,

-- 
Peter Xu