CVE-2026-89987

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 entry is created and

__filemap_remove_folio(folio, NULL) simply empties the i_pages slot. The

data is freed without ever being written to swap, and the next fault on

that index returns a freshly zeroed folio.

This is silent data loss for any process that keeps state in a MAP_SHARED

tmpfs segment across an unmap - for example a cache handed from one

process generation to the next through /dev/shm. It requires the folio to

be PMD-mapped, so it only shows up once shmem THP is enabled (which is

what we did in Meta fleet and started noticing crashes); with THP off the

pte path transfers the dirty bit correctly. It also only becomes visible

when swap is enabled, because with no swap device shmem folios (which are

on the anon LRU) are not scanned by reclaim at all, so the clean folio is

never dropped.

Reproduced on x86_64 with a tmpfs mounted huge=within_size: read-fault a

2MB-backed region, write a known pattern through the resulting mapping,

munmap, force reclaim of the cgroup, then re-map and read back. Without

this patch the region reads back as zeros and vmstat shows zswpout 0 - the

data was discarded rather than swapped. With this patch the region reads

back correctly and the pages are swapped out as expected. With

huge=never, or when the first touch is a write, the test passes either

way.