Executive brief
In the Linux kernel, the following vulnerability has been resolved:
mm/page_owner: use memcg_data snapshot to avoid TOCTOU in print_page_owner_memcg()
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.