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

[Andy Lutomirski]

On Sat, Dec 19, 2020 at 6:49 PM Andrea Arcangeli <aarcange@xxxxxxxxxx> wrote:
>
> On Sat, Dec 19, 2020 at 06:01:39PM -0800, Andy Lutomirski wrote:
> > I missed the beginning of this thread, but it looks to me like
> > userfaultfd changes PTEs with not locking except mmap_read_lock().  It
>
> There's no mmap_read_lock, I assume you mean mmap_lock for reading.

Yes.

>
> The ptes are changed always with the PT lock, in fact there's no
> problem with the PTE updates. The only difference with mprotect
> runtime is that the mmap_lock is taken for reading. And the effect
> contested for this change doesn't affect the PTE, but supposedly the
> tlb flushing deferral.

Can you point me at where the lock ends up being taken in this path?
I apparently missed it somewhere.

> Anyway to wait the wrprotect to do the deferred flush, before the
> unprotect can even start, one more mutex in the mm to take in all
> callers of change_protection_range with the mmap_lock for reading may
> be enough.

I'll read the code again tomorrow.