[PATCH 6.12.y] mm/ksm: fix userfaultfd protection during merge and unmerge

[Lance Yang <lance.yang@linux.dev>]

KSM loses userfaultfd protection when replace_page() builds a new PTE from
vma->vm_page_prot.  Dropping its uffd bit lets a subsequent write go
through ordinary COW instead of userfaultfd write protection handling.
For RWP, a merged page also becomes readable without faulting.

Keeping the uffd bit and restoring PAGE_NONE fixes merging, but we also
have to handle unmerging.  An existing KSM page can be protected with RWP
before MADV_UNMERGEABLE, so unmerging already needs fixing independently
of preserving RWP during merging.

Unsharing replaces a shared page with a private one and should leave
userfaultfd protection in place, as do_wp_page() already does for write
protection.  Currently, an unshare fault reaches do_uffd_rwp() before it
can proceed to do_wp_page().

- With synchronous RWP, handle_userfault() returns VM_FAULT_SIGBUS.
  break_ksm() then reports success with a KSM page still merged.

- With asynchronous RWP, protection is cleared before unsharing.

Preserve the uffd bit when merging, and restore PAGE_NONE for RWP.  Let
FAULT_FLAG_UNSHARE skip RWP fault handling and keep PAGE_NONE on each
private copy as well.  Unsharing can then complete with RWP still armed
for a subsequent access.

Link: https://lore.kernel.org/20261026130054.91461-1-lance.yang@linux.dev
Link: https://lore.kernel.org/all/20261019010934.697895-1-samarth.colleges@gmail.com/
Link: https://lore.kernel.org/all/20261020055518.36087-1-lance.yang@linux.dev/
Fixes: 63b2d4174c4a ("userfaultfd: wp: add the writeprotect API to userfaultfd ioctl")
Fixes: 2d427bb01639 ("mm/userfaultfd: add RWP fault delivery and expose UFFDIO_REGISTER_MODE_RWP")
Signed-off-by: Samarth Narang <samarth.colleges@gmail.com>
Signed-off-by: Lance Yang <lance.yang@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Co-developed-by: Samarth Narang <samarth.colleges@gmail.com>
Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Xu Xin <xu.xin@linux.dev>"
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Bobby Powers <bobbypowers@gmail.com>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Denis Plotnikov <dplotnikov@virtuozzo.com>
Cc: Doug Gilbert <dgilbert@interlog.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Martin Cracauer <cracauer@cons.org>
Cc: Marty McFadden <mcfadden8@llnl.gov>
Cc: Maya Gokhale <gokhale2@llnl.gov>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Shaohua Li <shli@fb.com>
Cc: James Houghton <jthoughton@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: SJ Park <sj@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Mohammed EL Kadiri <med08elkadiri@gmail.com>
Cc: <stable@vger.kernel.org>

(cherry picked from commit 990d6c2194a938a19a8cf94c6cfd75171345089b)
---
Backport to 6.12:
- Use the older pte_uffd_wp()/pte_mkuffd_wp() helpers.
- Drop the mm/memory.c changes. 6.12 has no RWP support.

 mm/ksm.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/ksm.c b/mm/ksm.c
index 15f558f69093..cbc666343b39 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1407,6 +1407,10 @@ static int replace_page(struct vm_area_struct *vma, struct page *page,
 		dec_mm_counter(mm, MM_ANONPAGES);
 	}
 
+	/* Preserve uffd-wp when replacing the page. */
+	if (pte_uffd_wp(orig_pte))
+		newpte = pte_mkuffd_wp(newpte);
+
 	flush_cache_page(vma, addr, pte_pfn(ptep_get(ptep)));
 	/*
 	 * No need to notify as we are replacing a read only page with another
-- 
2.49.0