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Subject: Re: [PATCH v26 24/30] x86/cet/shstk: Introduce shadow stack token
 setup/verify routines
To:     Eugene Syromiatnikov <esyr@redhat.com>
Cc:     Borislav Petkov <bp@alien8.de>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
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        linux-arch@vger.kernel.org, linux-api@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@kernel.org>,
        Balbir Singh <bsingharora@gmail.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Florian Weimer <fweimer@redhat.com>,
        "H.J. Lu" <hjl.tools@gmail.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>,
        Dave Martin <Dave.Martin@arm.com>,
        Weijiang Yang <weijiang.yang@intel.com>,
        Pengfei Xu <pengfei.xu@intel.com>,
        Haitao Huang <haitao.huang@intel.com>
References: <20210427204315.24153-1-yu-cheng.yu@intel.com>
 <20210427204315.24153-25-yu-cheng.yu@intel.com> <YKIfIEyW+sR+bDCk@zn.tnic>
 <e225e357-a1d5-9596-8900-79e6b94cf924@intel.com>
 <20210518001316.GR15897@asgard.redhat.com>
From:   "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
Message-ID: <ea096f7e-7761-081f-e855-3294f8d09471@intel.com>
Date:   Tue, 18 May 2021 11:05:25 -0700
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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