linux-kernel - Re: [PATCH v11 25/25] x86/cet/shstk: Add arch

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Date:   Tue, 01 Sep 2020 20:17:26 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
Cc:     "H.J. Lu" <hjl.tools@...il.com>, Dave Martin <Dave.Martin@....com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Weijiang Yang <weijiang.yang@...el.com>
Subject: Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for
 shadow stack

* Yu-cheng Yu:

> On 9/1/2020 10:50 AM, Florian Weimer wrote:
>> * Yu-cheng Yu:
>> 
>>> Like other arch_prctl()'s, this parameter was 'unsigned long'
>>> earlier. The idea was, since this arch_prctl is only implemented for
>>> the 64-bit kernel, we wanted it to look as 64-bit only.  I will change
>>> it back to 'unsigned long'.
>> What about x32?  In general, long is rather problematic for x32.
>
> The problem is the size of 'long', right?
> Because this parameter is passed in a register, and only the lower
> bits are used, x32 works as well.

The userspace calling convention leaves the upper 32-bit undefined.
Therefore, this only works by accident if the kernel does not check that
the upper 32-bit are zero, which is probably a kernel bug.

It's unclear to me what you are trying to accomplish.  Why do you want
to use unsigned long here?  The correct type appears to be unsigned int.
This correctly expresses that the upper 32 bits of the register do not
matter.

Thanks,
Florian