CVE-2026-43486: arm64: contpte: fix set_access_flags() no-op check for SMMU/ATS faults

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

arm64: contpte: fix setaccessflags() no-op check for SMMU/ATS faults

contpteptepsetaccessflags() compared the gathered ptepget() value against the requested entry to detect no-ops. ptepget() 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 PTERDONLY set in hardware.

For a CPU with FEATHAFDBS 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 PTERDONLY (needs PTERDONLY cleared) - read faults: target still lacks PTEAF

Fix by checking each sub-PTE against the requested AF/dirty/write state (the same bits consumed by ptepsetaccessflags()), using raw per-PTE values rather than the gathered ptepget() 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.