linux-kernel - Re: [PATCH v4 24/39] mm: Warn on shadow stack memory in wrong vma

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Date:   Fri, 2 Dec 2022 18:40:02 -0800
From:   Kees Cook <keescook@...omium.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>,
        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
Subject: Re: [PATCH v4 24/39] mm: Warn on shadow stack memory in wrong vma

On Fri, Dec 02, 2022 at 04:35:51PM -0800, Rick Edgecombe wrote:
> 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.
> 
> One sharp edge is that PTEs that are both Write=0 and Dirty=1 are
> treated as shadow by the CPU, but this combination used to be created by
> the kernel on x86. Previous patches have changed the kernel to now avoid
> creating these PTEs unless they are for shadow stack memory. In case any
> missed corners of the kernel are still creating PTEs like this for
> non-shadow stack memory, and to catch any re-introductions of the logic,
> warn if any shadow stack PTEs (Write=0, Dirty=1) are found in non-shadow
> stack VMAs when they are being zapped. This won't catch transient cases
> but should have decent coverage. It will be compiled out when shadow
> stack is not configured.
> 
> In order to check if a pte is shadow stack in core mm code, add default
> implmentations for pte_shstk() and pmd_shstk().
> 
> Tested-by: Pengfei Xu <pengfei.xu@...el.com>
> Tested-by: John Allen <john.allen@....com>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook