Fix for Loss of Young/Dirty Bits During Pagemap Scan

CVE-2024-36943

Summary

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

fs/proc/task_mmu: fix loss of young/dirty bits during pagemap scan

make_uffd_wp_pte() was previously doing:

pte = ptep_get(ptep); ptep_modify_prot_start(ptep); pte = pte_mkuffd_wp(pte); ptep_modify_prot_commit(ptep, pte);

But if another thread accessed or dirtied the pte between the first 2 calls, this could lead to loss of that information. Since ptep_modify_prot_start() gets and clears atomically, the following is the correct pattern and prevents any possible race. Any access after the first call would see an invalid pte and cause a fault:

pte = ptep_modify_prot_start(ptep); pte = pte_mkuffd_wp(pte); ptep_modify_prot_commit(ptep, pte);

References