When we try to unshare a pinned page for a private hugetlb, uffd-wp bit can
get lost during unsharing.  Fix it by carrying it over.

This should be very rare, only if an unsharing happened on a private
hugetlb page with uffd-wp protected (e.g. in a child which shares the same
page with parent with UFFD_FEATURE_EVENT_FORK enabled).

Cc: linux-stable <stable@vger.kernel.org>
Fixes: 166f3ecc0daf ("mm/hugetlb: hook page faults for uffd write protection")
Reported-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 mm/hugetlb.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 0213efaf31be..cd3a9d8f4b70 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5637,13 +5637,16 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
 	spin_lock(ptl);
 	ptep = hugetlb_walk(vma, haddr, huge_page_size(h));
 	if (likely(ptep && pte_same(huge_ptep_get(ptep), pte))) {
+		pte_t newpte = make_huge_pte(vma, &new_folio->page, !unshare);
+
 		/* Break COW or unshare */
 		huge_ptep_clear_flush(vma, haddr, ptep);
 		mmu_notifier_invalidate_range(mm, range.start, range.end);
 		page_remove_rmap(old_page, vma, true);
 		hugepage_add_new_anon_rmap(new_folio, vma, haddr);
-		set_huge_pte_at(mm, haddr, ptep,
-				make_huge_pte(vma, &new_folio->page, !unshare));
+		if (huge_pte_uffd_wp(pte))
+			newpte = huge_pte_mkuffd_wp(newpte);
+		set_huge_pte_at(mm, haddr, ptep, newpte);
 		folio_set_hugetlb_migratable(new_folio);
 		/* Make the old page be freed below */
 		new_folio = page_folio(old_page);
-- 
2.39.1

On Mon, 17 Apr 2023 15:53:13 -0400 Peter Xu <peterx@redhat.com> wrote:

> When we try to unshare a pinned page for a private hugetlb, uffd-wp bit can
> get lost during unsharing.  Fix it by carrying it over.
> 
> This should be very rare, only if an unsharing happened on a private
> hugetlb page with uffd-wp protected (e.g. in a child which shares the same
> page with parent with UFFD_FEATURE_EVENT_FORK enabled).

What are the user-visible consequences of the bug?

> Cc: linux-stable <stable@vger.kernel.org>

When proposing a backport, it's better to present the patch as a
standalone thing, against current -linus.  I'll then queue it in
mm-hotfixes and shall send it upstream during this -rc cycle.

As presented, this patch won't go upstream until after 6.3 is released,
and as it comes later in time, more backporting effort might be needed.

I can rework things if this fix is reasonably urgent (the "user-visible
consequences" info is the guide).  If not urgent, we can leave things
as they are.

Hi, Andrew,

On Mon, Apr 17, 2023 at 04:48:22PM -0700, Andrew Morton wrote:
> On Mon, 17 Apr 2023 15:53:13 -0400 Peter Xu <peterx@redhat.com> wrote:
> 
> > When we try to unshare a pinned page for a private hugetlb, uffd-wp bit can
> > get lost during unsharing.  Fix it by carrying it over.
> > 
> > This should be very rare, only if an unsharing happened on a private
> > hugetlb page with uffd-wp protected (e.g. in a child which shares the same
> > page with parent with UFFD_FEATURE_EVENT_FORK enabled).
> 
> What are the user-visible consequences of the bug?

When above condition met, one can lose uffd-wp bit on the privately mapped
hugetlb page.  It allows the page to be writable even if it should still be
wr-protected.  I assume it can mean data loss.

However it's very hard to trigger. When I wrote the reproducer (provided in
the last patch) I needed to use the newest gup_test cmd introduced by David
to trigger it because I don't even know another way to do a proper RO
longerm pin.

Besides that, it needs a bunch of other conditions all met:

        (1) hugetlb being mapped privately,
        (2) userfaultfd registered with WP and EVENT_FORK,
        (3) the user app fork()s, then,
        (4) RO longterm pin onto a wr-protected anonymous page.

If it's not impossible to hit in production I'd say extremely rare.

> 
> > Cc: linux-stable <stable@vger.kernel.org>
> 
> When proposing a backport, it's better to present the patch as a
> standalone thing, against current -linus.  I'll then queue it in
> mm-hotfixes and shall send it upstream during this -rc cycle.
> 
> As presented, this patch won't go upstream until after 6.3 is released,
> and as it comes later in time, more backporting effort might be needed.
> 
> I can rework things if this fix is reasonably urgent (the "user-visible
> consequences" info is the guide).  If not urgent, we can leave things
> as they are.

IMHO it's not urgent so suitable for mm-unstable (current base of this set;
sorry if I forgot to mention it explicitly).  I'll post (and remember to
post) patches on top of mm-stable if they're urgent, or e.g. bugs
introduced in current release.

I copied stable for the pure logic of fixing a bug in old kernels.  The
consequence of hitting the bug is very bad but chance to hit is very low.

Thanks,

-- 
Peter Xu