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Date:   Mon, 3 Oct 2022 15:49:18 -0700
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Subject: Re: [PATCH v2 22/39] mm: Don't allow write GUPs to shadow stack
 memory
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To:     Kees Cook <keescook@chromium.org>,
        Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc:     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>,
        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>,
        Florian Weimer <fweimer@redhat.com>,
        "H . J . Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        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>,
        "Ravi V . Shankar" <ravi.v.shankar@intel.com>,
        Weijiang Yang <weijiang.yang@intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        joao.moreira@intel.com, John Allen <john.allen@amd.com>,
        kcc@google.com, eranian@google.com, rppt@kernel.org,
        jamorris@linux.microsoft.com, dethoma@microsoft.com
References: <20220929222936.14584-1-rick.p.edgecombe@intel.com>
 <20220929222936.14584-23-rick.p.edgecombe@intel.com>
 <202210031134.B0B6B37@keescook>
From:   Andy Lutomirski <luto@kernel.org>
In-Reply-To: <202210031134.B0B6B37@keescook>
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On 10/3/22 11:39, Kees Cook wrote:
> On Thu, Sep 29, 2022 at 03:29:19PM -0700, Rick Edgecombe wrote:
>> [...]
>> Still allow FOLL_FORCE to write through shadow stack protections, as it
>> does for read-only protections.
> 
> As I asked in the cover letter: why do we need to add this for shstk? It
> was a mistake for general memory. :P

For debuggers, which use FOLL_FORCE, quite intentionally, to modify 
text.  And once a debugger has ptrace write access to a target, shadow 
stacks provide exactly no protection -- ptrace can modify text and all 
registers.

But /proc/.../mem may be a different story, and I'd be okay with having 
FOLL_PROC_MEM for legacy compatibility via /proc/.../mem and not 
allowing that to access shadow stacks.  This does seem like it may not 
be very useful, though.