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Subject: Re: [PATCH v27 24/31] x86/cet/shstk: Handle thread shadow stack
To:     Andy Lutomirski <luto@kernel.org>
Cc:     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>,
        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: <20210521221211.29077-1-yu-cheng.yu@intel.com>
 <20210521221211.29077-25-yu-cheng.yu@intel.com>
 <CALCETrVQ70VvZPf_tH5KOspYx6KwDg07dodzobBSABxMyc1xJw@mail.gmail.com>
From:   "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
Message-ID: <bae19cd9-e1ff-3ce4-f2e5-a12f34beb090@intel.com>
Date:   Tue, 25 May 2021 08:04:43 -0700
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On 5/22/2021 4:39 PM, Andy Lutomirski wrote:
> On Fri, May 21, 2021 at 3:14 PM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
>> diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
>> index 5ea2b494e9f9..8e5f772181b9 100644
>> --- a/arch/x86/kernel/shstk.c
>> +++ b/arch/x86/kernel/shstk.c
>> @@ -71,6 +71,53 @@ int shstk_setup(void)
>>          return 0;
>>   }
>>
>> +int shstk_alloc_thread_stack(struct task_struct *tsk, unsigned long clone_flags,
>> +                            unsigned long stack_size)
>> +{
> 
> ...
> 
>> +       state = get_xsave_addr(&tsk->thread.fpu.state.xsave, XFEATURE_CET_USER);
>> +       if (!state)
>> +               return -EINVAL;
>> +
> 
> The get_xsave_addr() API is horrible, and we already have one
> egregiously buggy instance in the kernel.  Let's not add another
> dubious use case.
> 
> If state == NULL, this means that CET_USER is in its init state.
> CET_USER in the init state should behave identically regardless of
> whether XINUSE[CET_USER] is set.  Can you please adjust this code
> accordingly?
> 

I will work on that.

Thanks,
Yu-cheng