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From:   "H.J. Lu" <hjl.tools@gmail.com>
Date:   Wed, 26 Aug 2020 12:43:39 -0700
Message-ID: <CAMe9rOprn69GcJ9btgDZA+ej3+vstNE0xd4wT27_2vcso0A4Og@mail.gmail.com>
Subject: Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for
 shadow stack
To:     "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Florian Weimer <fweimer@redhat.com>,
        Dave Martin <Dave.Martin@arm.com>,
        Dave Hansen <dave.hansen@intel.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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On Wed, Aug 26, 2020 at 11:49 AM Yu, Yu-cheng <yu-cheng.yu@intel.com> wrote:
>
> On 8/26/2020 10:04 AM, Andy Lutomirski wrote:
> > 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.
> >
>
> One benefit of MMAP_SPECIAL_* is there are more free bits than MAP_*.
> Does ARM have similar needs for memory mapping, Dave?
>

arch/arm64/include/uapi/asm/mman.h:

#define PROT_BTI   0x10     /* BTI guarded page */

-- 
H.J.