FAILED: patch "[PATCH] mm/ksm: fix userfaultfd protection during merge and unmerge" failed to apply to 6.18-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 6.18-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x 990d6c2194a938a19a8cf94c6cfd75171345089b
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261109200420-failed-6.18-990d6c2194a9@kernel.org' --subject-prefix 'PATCH 6.18.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 990d6c2194a938a19a8cf94c6cfd75171345089b Mon Sep 17 00:00:00 2001
From: Lance Yang <lance.yang@linux.dev>
Date: Mon, 26 Oct 2026 21:00:54 +0800
Subject: [PATCH] mm/ksm: fix userfaultfd protection during merge and unmerge
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

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>
---
 mm/ksm.c    |  8 +++++++-
 mm/memory.c | 14 ++++++++++----
 2 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 910a1307ac9f3..d6f736a1aa2f9 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -34,6 +34,7 @@
 #include <linux/memory.h>
 #include <linux/mmu_notifier.h>
 #include <linux/swap.h>
+#include <linux/userfaultfd_k.h>
 #include <linux/ksm.h>
 #include <linux/hashtable.h>
 #include <linux/freezer.h>
@@ -1463,9 +1464,14 @@ static int replace_page(struct vm_area_struct *vma, struct page *page,
 		dec_mm_counter(mm, MM_ANONPAGES);
 	}
 
-	/* Preserve soft-dirty when replacing the page. */
+	/* Preserve soft-dirty and uffd protection when replacing the page. */
 	if (pte_soft_dirty(orig_pte))
 		newpte = pte_mksoft_dirty(newpte);
+	if (pte_uffd(orig_pte)) {
+		newpte = pte_mkuffd(newpte);
+		if (userfaultfd_rwp(vma))
+			newpte = pte_modify(newpte, PAGE_NONE);
+	}
 
 	flush_cache_page(vma, addr, pte_pfn(ptep_get(ptep)));
 	/*
diff --git a/mm/memory.c b/mm/memory.c
index 916637e8381da..b65b56fe97148 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4125,8 +4125,11 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
 		if (unlikely(unshare)) {
 			if (pte_soft_dirty(vmf->orig_pte))
 				entry = pte_mksoft_dirty(entry);
-			if (pte_uffd(vmf->orig_pte))
+			if (pte_uffd(vmf->orig_pte)) {
 				entry = pte_mkuffd(entry);
+				if (userfaultfd_rwp(vma))
+					entry = pte_modify(entry, PAGE_NONE);
+			}
 		} else {
 			entry = maybe_mkwrite(pte_mkdirty(entry), vma);
 		}
@@ -6662,9 +6665,12 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
 		 * VM_UFFD_RWP VMA, a protnone PTE without the uffd bit is
 		 * NUMA hinting and must still fall through to do_numa_page().
 		 */
-		if (userfaultfd_pte_rwp(vmf->vma, vmf->orig_pte))
-			return do_uffd_rwp(vmf);
-		return do_numa_page(vmf);
+		if (userfaultfd_pte_rwp(vmf->vma, vmf->orig_pte)) {
+			if (!(vmf->flags & FAULT_FLAG_UNSHARE))
+				return do_uffd_rwp(vmf);
+		} else {
+			return do_numa_page(vmf);
+		}
 	}
 
 	spin_lock(vmf->ptl);
-- 
2.53.0