[PATCH v2 2/3] arm64: mm: use XN table mapping attributes for the linear region

[Anshuman Khandual]

On 3/8/21 11:45 PM, Ard Biesheuvel wrote:
> The way the arm64 kernel virtual address space is constructed guarantees
> that swapper PGD entries are never shared between the linear region on
> the one hand, and the vmalloc region on the other, which is where all
> kernel text, module text and BPF text mappings reside.
> 
> This means that mappings in the linear region (which never require
> executable permissions) never share any table entries at any level with
> mappings that do require executable permissions, and so we can set the
> table-level PXN attributes for all table entries that are created while
> setting up mappings in the linear region. Since swapper's PGD level page
> table is mapped r/o itself, this adds another layer of robustness to the
> way the kernel manages its own page tables. While at it, set the UXN
> attribute as well for all kernel mappings created at boot.

What happens when FEAT_HPDS is implemented and also being enabled ? Would
there be any adverse affect here or at least break the assumption that the
linear mapping page table entries are safer than before ? Hence, it might
be still better to check FEAT_HPDS feature enablement here, even it it is
not being used right now.