From fe623a81ad9eddf10059b4e44bca0440d9120613 Mon Sep 17 00:00:00 2001 From: Yafang Shao Date: Tue, 27 Oct 2026 10:17:20 +0800 Subject: [PATCH] mm/migrate: snapshot the source nid before the misplaced migration MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit A successful migration frees the source folio before the promotion counter update, so the folio_nid() read there operates on a freed folio — which may already have been reallocated. It happens to work today, as Zi explained: : Strictly speaking, it can be seen as a benign UAF. It works, because : 1. folio->flags happens to be the same as page->flags, : 2. folio/page nid is stored in the ->flags and never changed. : : In memdesc world, after a folio is freed, I assume folio->flags would be : zeroed since struct folio is freed and calling folio_nid() on a freed : folio would give a bogus result (yeah, it can be mitigated by not : zeroing node field and each node has its own struct folio pool, but that : makes folio_nid() implementation dependent). and Matthew explained further: : once struct folio is separately allocated, it's a UAF. It could be : recycled to a different NID or maybe the slab containing the folio could : be freed (assuming we're not holding the RCU read lock at this point in : migration, and I don't see why we would be). A separate concern is memory hot-removal. With memory-tiering mode enabled, successful migration can release the source before the promotion-accounting folio_nid() read. If the task is preempted there, an otherwise-free source block can be offlined and removed first. On the x86 path, removal can unmap the source's vmemmap, so the later read can fault. The snapshot avoids that read. Link: https://lore.kernel.org/20261027021721.95781-1-laoar.shao@gmail.com Fixes: e39bb6be9f2b ("NUMA Balancing: add page promotion counter") Signed-off-by: Yafang Shao Signed-off-by: Andrew Morton Suggested-by: Zi Yan Suggested-by: Andrew Morton Acked-by: Zi Yan Acked-by: David Hildenbrand (Arm) Cc: Ying Huang Cc: Matthew Wilcox Cc: Matthew Brost Cc: Joshua Hahn Cc: Byungchul Park Cc: Gregory Price Cc: Alistair Popple Cc: --- mm/migrate.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/mm/migrate.c b/mm/migrate.c index d649a6cfce6c..875079464930 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -2756,6 +2756,7 @@ int migrate_misplaced_folio(struct folio *folio, int node) pg_data_t *pgdat = NODE_DATA(node); int nr_remaining; unsigned int nr_succeeded; + int src_node = folio_nid(folio); LIST_HEAD(migratepages); struct mem_cgroup *memcg = get_mem_cgroup_from_folio(folio); struct lruvec *lruvec = mem_cgroup_lruvec(memcg, pgdat); @@ -2770,7 +2771,7 @@ int migrate_misplaced_folio(struct folio *folio, int node) count_vm_numa_events(NUMA_PAGE_MIGRATE, nr_succeeded); count_memcg_events(memcg, NUMA_PAGE_MIGRATE, nr_succeeded); if ((sysctl_numa_balancing_mode & NUMA_BALANCING_MEMORY_TIERING) - && !node_is_toptier(folio_nid(folio)) + && !node_is_toptier(src_node) && node_is_toptier(node)) mod_lruvec_state(lruvec, PGPROMOTE_SUCCESS, nr_succeeded); }