UBUNTU-CVE-2026-97945: Medium severity ubuntu/linux-hwe-edge vulnerability

In the Linux kernel, the following vulnerability has been resolved: x86/mm: Fix user-space data loss with MADVFREE and THP Some of users of Polars (a data analytics library) have lost production data from this bug. They seem to have just the right combination of huge pages, MADVFREE and heavy reclaim pressure. pmdmodify() masks the old value with (HPAGECHGMASK & ~PAGEDIRTY), silently discarding the hardware dirty bit. The subsequent pmdmksaveddirty() call is supposed to transfer PAGEDIRTY into PAGESAVEDDIRTY when write-protecting, but the dirty bit was already stripped from the value, so there is nothing left to transfer. Contrast with ptemodify(), which keeps PAGEDIRTYBITS in its mask, and pudmodify(), which keeps HPAGECHGMASK untouched: pmdmodify() is the odd one out. Any pmdmodify() on a writable, dirty PMD loses the dirty state. One visible consequence is data loss with MADVFREE on PMD-mapped THP: memset(buf, 0x5A, size); // PMD-mapped THP, PMD dirty madvise(buf, size, MADVFREE); // PMD cleaned but left writable, // folio marked lazyfree memset(buf, 0x5A, size); // hardware sets PAGEDIRTY again mprotect(buf, size, PROTREAD); // pmdmodify() drops the dirty bit mprotect(buf, size, PROTREAD|PROTWRITE); // ... memory pressure ... Reclaim (e.g. under memcg pressure) then finds the lazyfree folio with no dirty bit set anywhere and frees it in discardanonfoliopmdlocked(), even though the data was rewritten after MADVFREE; subsequent reads fault in fresh zero pages. NUMA hinting alone can trigger the same loss, as dohugepmdnumapage() restores the PMD through pmdmodify() as well. PMD-mapped file THPs are affected too: mprotect()/NUMA hinting dropping the dirty bit means rewritten data is never written back. Fix it by keeping PAGEDIRTY in the preserved mask, exactly like ptemodify() and pudmodify() do. The existing pmdmksaveddirty()/pmdclearsaveddirty() pair then performs the hardware-dirty <-> saved-dirty transition based on the write bit, preserving the shadow-stack encoding rules.