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

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Message-ID: <20230613074428.GS52412@kernel.org>
Date:   Tue, 13 Jun 2023 10:44:28 +0300
From:   Mike Rapoport <rppt@...nel.org>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc:     x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H . J . Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Allen <john.allen@....com>, kcc@...gle.com,
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        torvalds@...ux-foundation.org, broonie@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-s390@...r.kernel.org,
        xen-devel@...ts.xenproject.org
Subject: Re: [PATCH v9 02/42] mm: Move pte/pmd_mkwrite() callers with no VMA
 to _novma()

On Mon, Jun 12, 2023 at 05:10:28PM -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.
> 
> 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

I hear x86 maintainers asking to drop "previous patches" ;-)

Maybe
This is the second step of the conversion that moves the callers ...

> 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@...r.kernel.org
> Cc: linux-arm-kernel@...ts.infradead.org
> Cc: linux-s390@...r.kernel.org
> Cc: xen-devel@...ts.xenproject.org
> Cc: linux-arch@...r.kernel.org
> Cc: linux-mm@...ck.org
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>

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

-- 
Sincerely yours,
Mike.