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

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Date:   Fri, 17 Mar 2023 10:12:21 -0700
From:   Deepak Gupta <debug@...osinc.com>
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,
        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 Mon, Feb 27, 2023 at 2:31 PM Rick Edgecombe
<rick.p.edgecombe@...el.com> wrote:
>
> From: Yu-cheng Yu <yu-cheng.yu@...el.com>
>
> The x86 Control-flow Enforcement Technology (CET) 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.
>
> Account shadow stack pages to stack memory. Do this by adding a
> VM_SHADOW_STACK check in is_stack_mapping().
>
> Tested-by: Pengfei Xu <pengfei.xu@...el.com>
> Tested-by: John Allen <john.allen@....com>
> Tested-by: Kees Cook <keescook@...omium.org>
> Acked-by: Mike Rapoport (IBM) <rppt@...nel.org>
> Reviewed-by: Kees Cook <keescook@...omium.org>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
> Co-developed-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> Cc: Kees Cook <keescook@...omium.org>
>
> ---
> v7:
>  - Change is_stack_mapping() to know about VM_SHADOW_STACK so the
>    additions in vm_stat_account() can be dropped. (David Hildenbrand)
>
> v3:
>  - Remove unneeded VM_SHADOW_STACK check in accountable_mapping()
>    (Kirill)
>
> v2:
>  - Remove is_shadow_stack_mapping() and just change it to directly bitwise
>    and VM_SHADOW_STACK.
>
> Yu-cheng v26:
>  - Remove redundant #ifdef CONFIG_MMU.
>
> Yu-cheng v25:
>  - Remove #ifdef CONFIG_ARCH_HAS_SHADOW_STACK for is_shadow_stack_mapping().
> ---
>  mm/internal.h | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index 7920a8b7982e..1d13d5580f64 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -491,14 +491,14 @@ static inline bool is_exec_mapping(vm_flags_t flags)
>  }
>
>  /*
> - * 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.

>  }
>
>  /*
> --
> 2.17.1
>