linux-kernel - Re: [PATCH v26 24/30] x86/cet/shstk: Introduce shadow stack token setup/verify routines

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Date:   Tue, 18 May 2021 11:05:25 -0700
From:   "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To:     Eugene Syromiatnikov <esyr@...hat.com>
Cc:     Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.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>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>,
        Haitao Huang <haitao.huang@...el.com>
Subject: Re: [PATCH v26 24/30] x86/cet/shstk: Introduce shadow stack token
 setup/verify routines

On 5/17/2021 5:14 PM, Eugene Syromiatnikov wrote:
> On Mon, May 17, 2021 at 01:55:01PM -0700, Yu, Yu-cheng wrote:
>> On 5/17/2021 12:45 AM, Borislav Petkov wrote:
>>> On Tue, Apr 27, 2021 at 01:43:09PM -0700, Yu-cheng Yu wrote:
>>>> +static inline int write_user_shstk_32(u32 __user *addr, u32 val)
>>>> +{
>>>> +	WARN_ONCE(1, "%s used but not supported.\n", __func__);
>>>> +	return -EFAULT;
>>>> +}
>>>> +#endif
>>>
>>> What is that supposed to catch? Any concrete (mis-)use cases?
>>>
>>
>> If 32-bit apps are not supported, there should be no need of 32-bit shadow
>> stack write, otherwise there is a bug.
> 
> Speaking of which, I wonder what would happen if a 64-bit process makes
> a 32-bit system call (using int 0x80, for example), and gets a signal.
> 

Yes, that's right.  Thanks!  I should have said, if neither IA32 nor X32 
is supported.

Yu-cheng