[PATCH v9 01/42] mm: Rename arch pte_mkwrite()'s to pte_mkwrite_novma()

David Hildenbrand david at redhat.com
Tue Jun 13 22:26:53 AEST 2023

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.
> Start the process by renaming pte_mkwrite() to pte_mkwrite_novma() and
> adding the pte_mkwrite() wrapper in linux/pgtable.h. Apply the same
> pattern for pmd_mkwrite(). Since not all archs have a pmd_mkwrite_novma(),
> create a new arch config HAS_HUGE_PAGE that can be used to tell if
> pmd_mkwrite() should be defined. Otherwise in the !HAS_HUGE_PAGE cases the
> compiler would not be able to find pmd_mkwrite_novma().
> No functional change.
> Cc: linux-doc at vger.kernel.org
> Cc: linux-kernel at vger.kernel.org
> Cc: linux-alpha at vger.kernel.org
> Cc: linux-snps-arc at lists.infradead.org
> Cc: linux-arm-kernel at lists.infradead.org
> Cc: linux-csky at vger.kernel.org
> Cc: linux-hexagon at vger.kernel.org
> Cc: linux-ia64 at vger.kernel.org
> Cc: loongarch at lists.linux.dev
> Cc: linux-m68k at lists.linux-m68k.org
> Cc: Michal Simek <monstr at monstr.eu>
> Cc: Dinh Nguyen <dinguyen at kernel.org>
> Cc: linux-mips at vger.kernel.org
> Cc: openrisc at lists.librecores.org
> Cc: linux-parisc at vger.kernel.org
> Cc: linuxppc-dev at lists.ozlabs.org
> Cc: linux-riscv at lists.infradead.org
> Cc: linux-s390 at vger.kernel.org
> Cc: linux-sh at vger.kernel.org
> Cc: sparclinux at vger.kernel.org
> Cc: linux-um at lists.infradead.org
> Cc: linux-arch at vger.kernel.org
> Cc: linux-mm at kvack.org
> Suggested-by: Linus Torvalds <torvalds at linuxfoundation.org>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe at intel.com>
> Link: https://lore.kernel.org/lkml/CAHk-=wiZjSu7c9sFYZb3q04108stgHff2wfbokGCCgW7riz+8Q@mail.gmail.com/
> ---
> Hi Non-x86 Arch’s,
> x86 has a feature that allows for the creation of a special type of
> writable memory (shadow stack) that is only writable in limited specific
> ways. Previously, changes were proposed to core MM code to teach it to
> decide when to create normally writable memory or the special shadow stack
> writable memory, but David Hildenbrand suggested[0] to change
> pXX_mkwrite() to take a VMA, so awareness of shadow stack memory can be
> moved into x86 code. Later Linus suggested a less error-prone way[1] to go
> about this after the first attempt had a bug.
> Since pXX_mkwrite() is defined in every arch, it requires some tree-wide
> changes. So that is why you are seeing some patches out of a big x86
> series pop up in your arch mailing list. There is no functional change.
> After this refactor, the shadow stack series goes on to use the arch
> helpers to push arch memory details inside arch/x86 and other arch's
> with upcoming shadow stack features.
> Testing was just 0-day build testing.
> Hopefully that is enough context. Thanks!
> [0] https://lore.kernel.org/lkml/0e29a2d0-08d8-bcd6-ff26-4bea0e4037b0@redhat.com/
> [1] https://lore.kernel.org/lkml/CAHk-=wiZjSu7c9sFYZb3q04108stgHff2wfbokGCCgW7riz+8Q@mail.gmail.com/
> ---

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


David / dhildenb

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