6.12-stable review patch. If anyone has any objections, please let me know. ------------------ From: Ye Liu [ Upstream commit 90f095b816e25c6a9e4446d299bac5007fdcb3df ] print_page_owner_memcg() reads page->memcg_data via READ_ONCE() at the start to guard against tail pages and NULL data. However, it later re-reads page->memcg_data locklessly in two places: 1: page_memcg_check(page) 2: PageMemcgKmem(page) (via folio_memcg_kmem(), which includes VM_BUG_ON assertions for tail pages and MEMCG_DATA_OBJEXTS) If the page is concurrently freed and reallocated as a THP tail page or slab page between these calls, the VM_BUG_ON assertions can trigger on CONFIG_DEBUG_VM=y builds, crashing the kernel. Fix both TOCTOU issues by using the memcg_data snapshot throughout. Link: https://lore.kernel.org/20260714015117.78351-10-ye.liu@linux.dev Fixes: fcf8935832b8 ("mm/page_owner: print memcg information") Signed-off-by: Ye Liu Reported-by: Sashiko Reviewed-by: Zi Yan Reviewed-by: Vlastimil Babka (SUSE) Cc: Brendan Jackman Cc: Johannes Weiner Cc: Lorenzo Stoakes Cc: Michal Hocko Cc: Suren Baghdasaryan Cc: David Hildenbrand (Arm) Cc: Signed-off-by: Andrew Morton Signed-off-by: Sasha Levin --- mm/page_owner.c | 10 +++++++--- 1 file changed, 7 insertions(+), 3 deletions(-) diff --git a/mm/page_owner.c b/mm/page_owner.c index f92cec789eee5..5d1c946c0008f 100644 --- a/mm/page_owner.c +++ b/mm/page_owner.c @@ -501,6 +501,7 @@ static inline int print_page_owner_memcg(char *kbuf, size_t count, int ret, { #ifdef CONFIG_MEMCG unsigned long memcg_data; + struct obj_cgroup *objcg; struct mem_cgroup *memcg; bool online; char name[80]; @@ -510,11 +511,14 @@ static inline int print_page_owner_memcg(char *kbuf, size_t count, int ret, if (!memcg_data) goto out_unlock; - if (memcg_data & MEMCG_DATA_OBJEXTS) + if (memcg_data & MEMCG_DATA_OBJEXTS) { ret += scnprintf(kbuf + ret, count - ret, "Slab cache page\n"); + goto out_unlock; + } - memcg = page_memcg_check(page); + objcg = (void *)(memcg_data & ~OBJEXTS_FLAGS_MASK); + memcg = objcg ? obj_cgroup_memcg(objcg) : NULL; if (!memcg) goto out_unlock; @@ -522,7 +526,7 @@ static inline int print_page_owner_memcg(char *kbuf, size_t count, int ret, cgroup_name(memcg->css.cgroup, name, sizeof(name)); ret += scnprintf(kbuf + ret, count - ret, "Charged %sto %smemcg %s\n", - PageMemcgKmem(page) ? "(via objcg) " : "", + (memcg_data & MEMCG_DATA_KMEM) ? "(via objcg) " : "", online ? "" : "offline ", name); out_unlock: -- 2.53.0