Affected: Linux

In the Linux kernel, the following vulnerability has been resolved:

mm/huge_memory: transfer the pmd dirty bit to the folio on zap

zap_huge_pmd_folio() propagates the pmd young bit to the folio for the file case, but not the dirty bit. The pte path does propagate it, in zap_present_folio_ptes() and so does the pmd split path, in __split_huge_pmd_locked().

For most file mappings the omission is harmless, because writing to a shared file mapping goes through page_mkwrite(), which dirties the folio. tmpfs is different: it has no page_mkwrite(), and vma_wants_writenotify() is false for it, so a *read* fault on a MAP_SHARED tmpfs mapping installs a writable pmd via do_read_fault(). do_read_fault() does not call fault_dirty_shared_page(), so subsequent stores through that mapping set only the hardware dirty bit in the pmd and never call folio_mark_dirty(). A shmem folio allocated by a fault is marked uptodate but not dirty (see the clear: block in shmem_get_folio_gfp()), so PG_dirty is never set at all.

Unmapping such a folio - munmap(), or exit_mmap() when the process dies - then loses the only record that it was written, because zap_huge_pmd() drops the pmd without transferring the dirty bit. Reclaim afterwards sees a clean shmem folio: the whole swap-out block in shrink_folio_list() is inside "if (folio_test_dirty(folio))", so pageout() is skipped and the folio falls into __remove_mapping(). There, folio_is_file_lru() is false for a swapbacked folio, so no shadow entr…