Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect

On Mon, Dec 21, 2020 at 12:23:06PM -0800, Nadav Amit wrote:

2. Copy the page in cow_user_page() while holding the PTL and after flushing has been done. I am not sure if there are potential problems with special-pages (2 flushes might be necessary for special pages).

This seems to be a good thing irrelevant of userfaultfd.

Frankly speaking I don't know why it's always safe to do data copy without the pgtable lock in wp_page_copy(), since I don't know what guaranteed us from data changing on the original page due to any reason. We check and make sure pte hasn't changed, but is that really enough?

So the safe way to me is that wp_page_copy() not only check pte_same() but also data_same() on the old/new pages, then with that it's even easier if we just move the data copy into the ptl as what Nadav said here. But I have no idea on the penalty to the rest of the world, especially on non-modern hosts.

Thanks,