FAILED: patch "[PATCH] mm/hugetlb: fix use-after-free in hugetlb_mfill_atomic_pte()" failed to apply to 5.15-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 5.15-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-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x d8a9cd7fd2fb01211430c933932da58839b8ca65
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026110447-cucumber-tummy-2f42@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Possible dependencies:



thanks,

greg k-h

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

From d8a9cd7fd2fb01211430c933932da58839b8ca65 Mon Sep 17 00:00:00 2001
From: "David Hildenbrand (Arm)" <david@kernel.org>
Date: Thu, 15 Oct 2026 18:07:09 +0200
Subject: [PATCH] mm/hugetlb: fix use-after-free in hugetlb_mfill_atomic_pte()

Let's fix another instance where we use a folio after dropping the PTL,
where someone else could e.g., MADV_DONTNEED it once we drop the PTL,
making us trigger a use-after-free on the folio.

Link: https://lore.kernel.org/20261015-urgent_fixes-v1-3-42796911043b@kernel.org
Fixes: cb6acd01e2e4 ("hugetlbfs: fix races and page leaks during migration")
Fixes: 1c9e8def43a3 ("userfaultfd: hugetlbfs: add UFFDIO_COPY support for shared mappings")
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reported-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Acked-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Naoya Horiguchi <nao.horiguchi@gmail.com>
Cc: Adam Litke <agl@us.ibm.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Rik van Riel <riel@surriel.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Kiryl Shutsemau <kas@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: <stable@vger.kernel.org>

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 16a3abb985be..5db64fa98404 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6520,11 +6520,16 @@ int hugetlb_mfill_atomic_pte(pte_t *dst_pte,
 	/* No need to invalidate - it was non-present before */
 	update_mmu_cache(dst_vma, dst_addr, dst_pte);
 
-	spin_unlock(ptl);
 	if (!is_continue)
 		folio_set_hugetlb_migratable(folio);
 	if (vm_shared || is_continue)
 		folio_unlock(folio);
+
+	/*
+	 * Hold the PTL until we're done with the folio: the page table owns the
+	 * folio reference now.
+	 */
+	spin_unlock(ptl);
 	ret = 0;
 out:
 	return ret;