In the Linux kernel, the following vulnerability has been resolved:
arm64: contpte: fix set_access_flags() no-op check for SMMU/ATS faults
contpte_ptep_set_access_flags() compared the gathered ptep_get() value
against the requested entry to detect no-ops. ptep_get() ORs AF/dirty
from all sub-PTEs in the CONT block, so a dirty sibling can make the
target appear already-dirty. When the gathered value matches entry, the
function returns 0 even though the target sub-PTE still has PTE_RDONLY
set in hardware.
For a CPU with FEAT_HAFDBS this gathered view is fine, since hardware may
set AF/dirty on any sub-PTE and CPU TLB behavior is effectively gathered
across the CONT range. But page-table walkers that evaluate each
descriptor individually (e.g. a CPU without DBM support, or an SMMU
without HTTU, or with HA/HD disabled in CD.TCR) can keep faulting on the
unchanged target sub-PTE, causing an infinite fault loop.
Gathering can therefore cause false no-ops when only a sibling has been
updated:
- write faults: target still has PTE_RDONLY (needs PTE_RDONLY cleared)
- read faults: target still lacks PTE_AF
Fix by checking each sub-PTE against the requested AF/dirty/write state
(the same bits consumed by __ptep_set_access_flags()), using raw
per-PTE values rather than the gathered ptep_get() view, before
returning no-op. Keep using the raw target PTE for the write-bit unfold
decision.
Per Arm ARM (DDI 0487) D8.7.1 ("The Contiguous bit"), any sub-PTE in a CONT
range may become the effective cached translation and software must
maintain consistent attributes across the range.