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Date:   Tue, 13 Jun 2023 10:43:47 +0300
From:   Mike Rapoport <rppt@kernel.org>
To:     Rick Edgecombe <rick.p.edgecombe@intel.com>
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Subject: Re: [PATCH v9 01/42] mm: Rename arch pte_mkwrite()'s to
 pte_mkwrite_novma()
Message-ID: <20230613074347.GR52412@kernel.org>
References: <20230613001108.3040476-1-rick.p.edgecombe@intel.com>
 <20230613001108.3040476-2-rick.p.edgecombe@intel.com>
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On Mon, Jun 12, 2023 at 05:10:27PM -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:                            ^ mapping?

> 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@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-alpha@vger.kernel.org
> Cc: linux-snps-arc@lists.infradead.org
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-csky@vger.kernel.org
> Cc: linux-hexagon@vger.kernel.org
> Cc: linux-ia64@vger.kernel.org
> Cc: loongarch@lists.linux.dev
> Cc: linux-m68k@lists.linux-m68k.org
> Cc: Michal Simek <monstr@monstr.eu>
> Cc: Dinh Nguyen <dinguyen@kernel.org>
> Cc: linux-mips@vger.kernel.org
> Cc: openrisc@lists.librecores.org
> Cc: linux-parisc@vger.kernel.org
> Cc: linuxppc-dev@lists.ozlabs.org
> Cc: linux-riscv@lists.infradead.org
> Cc: linux-s390@vger.kernel.org
> Cc: linux-sh@vger.kernel.org
> Cc: sparclinux@vger.kernel.org
> Cc: linux-um@lists.infradead.org
> Cc: linux-arch@vger.kernel.org
> Cc: linux-mm@kvack.org
> Suggested-by: Linus Torvalds <torvalds@linuxfoundation.org>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> Link: https://lore.kernel.org/lkml/CAHk-=wiZjSu7c9sFYZb3q04108stgHff2wfbokGCCgW7riz+8Q@mail.gmail.com/

Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>

-- 
Sincerely yours,
Mike.