Re: [PATCH] userfaultfd: provide properly masked address for huge-pages

[Peter Xu <peterx@redhat.com>]

On Mon, Jul 11, 2022 at 09:59:06AM -0700, Nadav Amit wrote:
> From: Nadav Amit <namit@vmware.com>
> 
> Commit 824ddc601adc ("userfaultfd: provide unmasked address on
> page-fault") was introduced to fix an old bug, in which the offset in
> the address of a page-fault was masked. Concerns were raised - although
> were never backed by actual code - that some userspace code might break
> because the bug has been around for quite a while. To address these
> concerns a new flag was introduced, and only when this flag is set by
> the user, userfaultfd provides the exact address of the page-fault.
> 
> The commit however had a bug, and if the flag is unset, the offset was
> always masked based on a base-page granularity. Yet, for huge-pages, the
> behavior prior to the commit was that the address is masked to the
> huge-page granulrity.
> 
> While there are no reports on real breakage, fix this issue. If the flag
> is unset, use the address with the masking that was done before.
> 
> Fixes: 824ddc601adc ("userfaultfd: provide unmasked address on page-fault")
> Reported-by: James Houghton <jthoughton@google.com>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Mike Rapoport <rppt@linux.ibm.com>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Nadav Amit <namit@vmware.com>

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

-- 
Peter Xu