Re: [PATCH v9 02/42] mm: Move pte/pmd_mkwrite() callers with no VMA to _novma()

David Hildenbrand
2023-06-13T12:29:24Z

On 13.06.23 02:10, Rick Edgecombe wrote:
> The x86 Shadow stack feature includes a new type of memory called shadow
> stack. This shadow stack memory has some unusual properties, which requires
> some core mm changes to function properly.
>
> One of these unusual properties is that shadow stack memory is writable,
> but only in limited ways. These limits are applied via a specific PTE
> bit combination. Nevertheless, the memory is writable, and core mm code
> will need to apply the writable permissions in the typical paths that
> call pte_mkwrite(). Future patches will make pte_mkwrite() take a VMA, so
> that the x86 implementation of it can know whether to create regular
> writable memory or shadow stack memory.
>
> But there are a couple of challenges to this. Modifying the signatures of
> each arch pte_mkwrite() implementation would be error prone because some
> are generated with macros and would need to be re-implemented. Also, some
> pte_mkwrite() callers operate on kernel memory without a VMA.
>
> So this can be done in a three step process. First pte_mkwrite() can be
> renamed to pte_mkwrite_novma() in each arch, with a generic pte_mkwrite()
> added that just calls pte_mkwrite_novma(). Next callers without a VMA can
> be moved to pte_mkwrite_novma(). And lastly, pte_mkwrite() and all callers
> can be changed to take/pass a VMA.
>
> Previous patches have done the first step, so next move the callers that
> don't have a VMA to pte_mkwrite_novma(). Also do the same for
> pmd_mkwrite(). This will be ok for the shadow stack feature, as these
> callers are on kernel memory which will not need to be made shadow stack,
> and the other architectures only currently support one type of memory
> in pte_mkwrite()
>
> Cc: linux-doc@vger.kernel.org
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-s390@vger.kernel.org
> Cc: xen-devel@lists.xenproject.org
> Cc: linux-arch@vger.kernel.org
> Cc: linux-mm@kvack.org
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> ---

Acked-by: David Hildenbrand <david@redhat.com>

--
Cheers,

David / dhildenb