FAILED: patch "[PATCH] mm/page_isolation: guard compound_order() against racing" failed to apply to 6.12-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 6.12-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-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x 4c767c5dd9bc3521fed11e6fe1d52621fe5abf7b
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107051925-failed-6.12-4c767c5dd9bc@kernel.org' --subject-prefix 'PATCH 6.12.y' 'HEAD^..'

Thanks,
Sasha

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

From 4c767c5dd9bc3521fed11e6fe1d52621fe5abf7b Mon Sep 17 00:00:00 2001
From: Qi Xi <xiqi2@huawei.com>
Date: Tue, 25 Aug 2026 20:05:49 +0800
Subject: [PATCH] mm/page_isolation: guard compound_order() against racing

The PageCompound branch reads compound_head() without holding a reference.
A racing split or free can cause compound_head() to return a stale
pointer, and compound_nr() reads the order from that stale head, leading
to out-of-range shifts and making the skip distance meaningless.

Read the order explicitly with compound_order() and validate it is within
MAX_FOLIO_ORDER before shifting.  Also verify the derived head_pfn against
the legitimate pfn: the head must not be past pfn, must be aligned to
nr_pages, and pfn must fall within the compound page.  Bail out with
-EBUSY if any check fails.

Link: https://lore.kernel.org/20260825120549.966271-3-xiqi2@huawei.com
Fixes: b2c9e2fbba32 ("mm: make alloc_contig_range work at pageblock granularity")
Signed-off-by: Qi Xi <xiqi2@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Suggested-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Cc: Brendan Jackman <brendan.jackman@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Nanyong Sun <sunnanyong@huawei.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: <stable@vger.kernel.org>
---
 mm/page_isolation.c | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index e4ee998c00f3b..5aaad037384e4 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -419,10 +419,28 @@ static int isolate_single_pageblock(unsigned long boundary_pfn,
 		if (PageCompound(page)) {
 			struct page *head = compound_head(page);
 			unsigned long head_pfn = page_to_pfn(head);
-			unsigned long nr_pages = compound_nr(head);
+			unsigned int order = compound_order(head);
+			unsigned long nr_pages;
+
+			/* compound_order() is racy. Cap it at MAX_FOLIO_ORDER. */
+			if (order > MAX_FOLIO_ORDER)
+				goto failed;
+
+			nr_pages = 1UL << order;
+
+			/*
+			 * compound_head() is also racy, so the derived head_pfn
+			 * needs additional checks to make sure it is valid.
+			 * Otherwise, just fail the check. pfn comes from
+			 * __first_valid_page() as a legitimate PFN, so use it to
+			 * check head_pfn.
+			 */
+			if (head_pfn > pfn || !IS_ALIGNED(head_pfn, nr_pages) ||
+			    pfn - head_pfn >= nr_pages)
+				goto failed;
 
 			if (head_pfn + nr_pages <= boundary_pfn ||
-			    PageHuge(page)) {
+			    PageHuge(head)) {
 				pfn = head_pfn + nr_pages;
 				continue;
 			}
-- 
2.53.0