From:   Florian Weimer <fweimer@redhat.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Rick Edgecombe <rick.p.edgecombe@intel.com>, x86@kernel.org,
        "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-mm@kvack.org,
        linux-arch@vger.kernel.org, linux-api@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@kernel.org>,
        Balbir Singh <bsingharora@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Eugene Syromiatnikov <esyr@redhat.com>,
        "H . J . Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <keescook@chromium.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        Weijiang Yang <weijiang.yang@intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        John Allen <john.allen@amd.com>, kcc@google.com,
        eranian@google.com, rppt@kernel.org, jamorris@linux.microsoft.com,
        dethoma@microsoft.com, akpm@linux-foundation.org,
        Andrew.Cooper3@citrix.com, christina.schimpe@intel.com
Subject: Re: [PATCH v5 23/39] mm: Don't allow write GUPs to shadow stack memory
References: <20230119212317.8324-1-rick.p.edgecombe@intel.com>
        <20230119212317.8324-24-rick.p.edgecombe@intel.com>
        <aa973c0f-5d90-36df-01b2-db9d9182910e@redhat.com>
Date:   Mon, 23 Jan 2023 11:45:04 +0100
In-Reply-To: <aa973c0f-5d90-36df-01b2-db9d9182910e@redhat.com> (David
        Hildenbrand's message of "Mon, 23 Jan 2023 10:10:04 +0100")
Message-ID: <87fsc1il73.fsf@oldenburg.str.redhat.com>
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* David Hildenbrand:

> On 19.01.23 22:23, 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.
>> Shadow stack memory is writable only in very specific, controlled
>> ways.
>> However, since it is writable, the kernel treats it as such. As a result
>> there remain many ways for userspace to trigger the kernel to write to
>> shadow stack's via get_user_pages(, FOLL_WRITE) operations. To make this a
>> little less exposed, block writable GUPs for shadow stack VMAs.
>> Still allow FOLL_FORCE to write through shadow stack protections, as
>> it
>> does for read-only protections.
>
> So an app can simply modify the shadow stack itself by writing to
> /proc/self/mem ?
>
> Is that really intended? Looks like security hole to me at first
> sight, but maybe I am missing something important.

Isn't it possible to overwrite GOT pointers using the same vector?
So I think it's merely reflecting the status quo.

Thanks,
Florian