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From:   Andy Lutomirski <luto@kernel.org>
Date:   Wed, 26 Aug 2020 10:04:53 -0700
Message-ID: <CALCETrVeNA0Kt2rW0CRCVo1JE0CKaBxu9KrJiyqUA8LPraY=7g@mail.gmail.com>
Subject: Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for
 shadow stack
To:     Florian Weimer <fweimer@redhat.com>
Cc:     Dave Martin <Dave.Martin@arm.com>,
        "Yu, Yu-cheng" <yu-cheng.yu@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Andy Lutomirski <luto@kernel.org>, 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>,
        "H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
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        Kees Cook <keescook@chromium.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
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        Randy Dunlap <rdunlap@infradead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
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        Weijiang Yang <weijiang.yang@intel.com>
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On Wed, Aug 26, 2020 at 9:52 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> * Dave Martin:
>
> > On Tue, Aug 25, 2020 at 04:34:27PM -0700, Yu, Yu-cheng wrote:
> >> On 8/25/2020 4:20 PM, Dave Hansen wrote:
> >> >On 8/25/20 2:04 PM, Yu, Yu-cheng wrote:
> >> >>>>I think this is more arch-specific.  Even if it becomes a new syscall,
> >> >>>>we still need to pass the same parameters.
> >> >>>
> >> >>>Right, but without the copying in and out of memory.
> >> >>>
> >> >>Linux-api is already on the Cc list.  Do we need to add more people to
> >> >>get some agreements for the syscall?
> >> >What kind of agreement are you looking for?  I'd suggest just coding it
> >> >up and posting the patches.  Adding syscalls really is really pretty
> >> >straightforward and isn't much code at all.
> >> >
> >>
> >> Sure, I will do that.
> >
> > Alternatively, would a regular prctl() work here?
>
> Is this something appliation code has to call, or just the dynamic
> loader?
>
> prctl in glibc is a variadic function, so if there's a mismatch between
> the kernel/userspace syscall convention and the userspace calling
> convention (for variadic functions) for specific types, it can't be made
> to work in a generic way.
>
> The loader can use inline assembly for system calls and does not have
> this issue, but applications would be implcated by it.
>

I would expect things like Go and various JITs to call it directly.

If we wanted to be fancy and add a potentially more widely useful
syscall, how about:

mmap_special(void *addr, size_t length, int prot, int flags, int type);

Where type is something like MMAP_SPECIAL_X86_SHSTK.  Fundamentally,
this is really just mmap() except that we want to map something a bit
magical, and we don't want to require opening a device node to do it.

--Andy