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Subject: Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for
 shadow stack
To:     Dave Martin <Dave.Martin@arm.com>, "H.J. Lu" <hjl.tools@gmail.com>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Florian Weimer <fweimer@redhat.com>, X86 ML <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
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        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>,
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        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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 <20200901102758.GY6642@arm.com>
From:   "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
Message-ID: <c91bbad8-9e45-724b-4526-fe3674310c57@intel.com>
Date:   Tue, 1 Sep 2020 10:23:11 -0700
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On 9/1/2020 3:28 AM, Dave Martin wrote:
> On Thu, Aug 27, 2020 at 06:26:11AM -0700, H.J. Lu wrote:
>> On Wed, Aug 26, 2020 at 12:57 PM Dave Hansen <dave.hansen@intel.com> wrote:
>>>
>>> On 8/26/20 11:49 AM, Yu, Yu-cheng wrote:
>>>>> 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?
>>>
>>> No idea.
>>>
>>> But, mmap_special() is *basically* mmap2() with extra-big flags space.
>>> I suspect it will grow some more uses on top of shadow stacks.  It could
>>> have, for instance, been used to allocate MPX bounds tables.
>>
>> There is no reason we can't use
>>
>> long arch_prctl (int, unsigned long, unsigned long, unsigned long, ..);
>>
>> for ARCH_X86_CET_MMAP_SHSTK.   We just need to use
>>
>> syscall (SYS_arch_prctl, ARCH_X86_CET_MMAP_SHSTK, ...);
> 
> 
> 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?

> 
> For now, I can't see what arg[2] is used for (and hence the type
> argument of mmap_special()), but I haven't dug through the whole series.

If we use the approach above, then we don't need arch_prctl changes.

Thanks,
Yu-cheng