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Subject: Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for
 shadow stack
To:     Florian Weimer <fweimer@redhat.com>,
        "H.J. Lu" <hjl.tools@gmail.com>
Cc:     Dave Martin <Dave.Martin@arm.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>,
        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:   "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
Message-ID: <a770d45d-b147-a8c5-b7f8-30d668cbed84@intel.com>
Date:   Thu, 27 Aug 2020 11:13:48 -0700
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On 8/27/2020 6:36 AM, Florian Weimer wrote:
> * H. J. Lu:
> 
>> On Thu, Aug 27, 2020 at 6:19 AM Florian Weimer <fweimer@redhat.com> wrote:
>>>
>>> * Dave Martin:
>>>
>>>> You're right that this has implications: for i386, libc probably pulls
>>>> more arguments off the stack than are really there in some situations.
>>>> This isn't a new problem though.  There are already generic prctls with
>>>> fewer than 4 args that are used on x86.
>>>
>>> As originally posted, glibc prctl would have to know that it has to pull
>>> an u64 argument off the argument list for ARCH_X86_CET_DISABLE.  But
>>> then the u64 argument is a problem for arch_prctl as well.
>>>
>>
>> Argument of ARCH_X86_CET_DISABLE is int and passed in register.
> 
> The commit message and the C source say otherwise, I think (not sure
> about the C source, not a kernel hacker).
> 

H.J. Lu suggested that we fix x86 arch_prctl() to take four arguments, 
and then keep MMAP_SHSTK as an arch_prctl().  Because now the map flags 
and size are all in registers, this also solves problems being pointed 
out earlier.  Without a wrapper, the shadow stack mmap call (from user 
space) will be:

syscall(_NR_arch_prctl, ARCH_X86_CET_MMAP_SHSTK, size, MAP_32BIT).

I think this would be a nice alternative to another new syscall.

If this looks good to everyone, I can send out new patches as response 
to my current version, and then after all issues fixed, send v12.

Thanks,
Yu-cheng