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Subject: Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for
 shadow stack
To:     Andy Lutomirski <luto@kernel.org>,
        "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
Cc:     Dave Martin <Dave.Martin@arm.com>, "H.J. Lu" <hjl.tools@gmail.com>,
        Florian Weimer <fweimer@redhat.com>, X86 ML <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux API <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>,
        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>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
        Weijiang Yang <weijiang.yang@intel.com>
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From:   Dave Hansen <dave.hansen@intel.com>
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Message-ID: <32005d57-e51a-7c7f-4e86-612c2ff067f3@intel.com>
Date:   Tue, 1 Sep 2020 11:11:37 -0700
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On 9/1/20 10:45 AM, Andy Lutomirski wrote:
>>> For arm64 (and sparc etc.) we continue to use the regular mmap/mprotect
>>> family of calls.  One or two additional arch-specific mmap flags are
>>> sufficient for now.
>>>
>>> Is x86 definitely not going to fit within those calls?
>> That can work for x86.  Andy, what if we create PROT_SHSTK, which can
>> been seen only from the user.  Once in kernel, it is translated to
>> VM_SHSTK.  One question for mremap/mprotect is, do we allow a normal
>> data area to become shadow stack?
> I'm unconvinced that we want to use a somewhat precious PROT_ or VM_
> bit for this.  Using a flag bit makes sense if we expect anyone to
> ever map an fd or similar as a shadow stack, but that seems a bit odd
> in the first place.  To me, it seems more logical for a shadow stack
> to be a special sort of mapping with a special vm_ops, not a normal
> mapping with a special flag set.  Although I realize that we want
> shadow stacks to work like anonymous memory with respect to fork().
> Dave?

I actually don't like the idea of *creating* mappings much.

I think the pkey model has worked out pretty well where we separate
creating the mapping from doing something *to* it, like changing
protections.  For instance, it would be nice if we could preserve things
like using hugetlbfs or heck even doing KSM for shadow stacks.

If we're *creating* mappings, we've pretty much ruled out things like
hugetlbfs.

Something like mprotect_shstk() would allow an implementation today that
only works on anonymous memory *and* sets up a special vm_ops.  But, the
same exact ABI could do wonky stuff in the future if we decided we
wanted to do shadow stacks on DAX or hugetlbfs or whatever.

I don't really like the idea of PROT_SHSTK those are plumbed into a
bunch of interfaces.  But, I also can't deny that it seems to be working
fine for the arm64 folks.