linux-kernel - Re: [PATCH v7 22/41] mm/mmap: Add shadow stack pages to memory accounting

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Message-ID: <236ae66c-fafb-80e9-d58b-6b18a22071c1@intel.com>
Date:   Fri, 17 Mar 2023 10:16:18 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Deepak Gupta <debug@...osinc.com>,
        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,
        eranian@...gle.com, rppt@...nel.org, jamorris@...ux.microsoft.com,
        dethoma@...rosoft.com, akpm@...ux-foundation.org,
        Andrew.Cooper3@...rix.com, christina.schimpe@...el.com,
        david@...hat.com, Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v7 22/41] mm/mmap: Add shadow stack pages to memory
 accounting

On 3/17/23 10:12, Deepak Gupta wrote:
>>  /*
>> - * Stack area - automatically grows in one direction
>> + * Stack area
>>   *
>> - * VM_GROWSUP / VM_GROWSDOWN VMAs are always private anonymous:
>> - * do_mmap() forbids all other combinations.
>> + * VM_GROWSUP, VM_GROWSDOWN VMAs are always private
>> + * anonymous. do_mmap() forbids all other combinations.
>>   */
>>  static inline bool is_stack_mapping(vm_flags_t flags)
>>  {
>> -       return (flags & VM_STACK) == VM_STACK;
>> +       return ((flags & VM_STACK) == VM_STACK) || (flags & VM_SHADOW_STACK);
> Same comment here. `VM_SHADOW_STACK` is an x86 specific way of
> encoding a shadow stack.
> Instead let's have a proxy here which allows architectures to have
> their own encodings to represent a shadow stack.

This doesn't _preclude_ another architecture from coming along and doing
that, right?  I'd just prefer that shadow stack architecture #2 comes
along and refactors this in precisely the way _they_ need it.