Re: [PATCH v9 03/42] mm: Make pte_mkwrite() take a VMA

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On Mon, Jun 12, 2023 at 05:10:29PM -0700, 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.

Nit:                             ^ mappings?
> 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.
> In a previous patches, pte_mkwrite() was renamed pte_mkwrite_novma() and
> callers that don't have a VMA were changed to use pte_mkwrite_novma(). So

This is the third step so pte_mkwrite() was renamed to pte_mkwrite_novma()
and ...

> now change pte_mkwrite() to take a VMA and change the remaining callers to
> pass a VMA. Apply the same changes for pmd_mkwrite().
> No functional change.
> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>

Reviewed-by: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>

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