linux-kernel - Re: [PATCH v27 23/31] x86/cet/shstk: Add user-mode shadow stack support

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Date:   Tue, 20 Jul 2021 10:13:43 -0700
From:   "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "Xu, Pengfei" <pengfei.xu@...el.com>,
        "vedvyas.shanbhogue@...el.com" <vedvyas.shanbhogue@...el.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "nadav.amit@...il.com" <nadav.amit@...il.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "corbet@....net" <corbet@....net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jannh@...gle.com" <jannh@...gle.com>,
        "x86@...nel.org" <x86@...nel.org>, "bp@...en8.de" <bp@...en8.de>,
        "pavel@....cz" <pavel@....cz>,
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        "keescook@...omium.org" <keescook@...omium.org>,
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        "Huang, Haitao" <haitao.huang@...el.com>,
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        "Yang, Weijiang" <weijiang.yang@...el.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v27 23/31] x86/cet/shstk: Add user-mode shadow stack
 support

On 7/19/2021 11:23 AM, Edgecombe, Rick P wrote:
> On Fri, 2021-05-21 at 15:12 -0700, Yu-cheng Yu wrote:
>> Introduce basic shadow stack enabling/disabling/allocation routines.
>> A task's shadow stack is allocated from memory with VM_SHADOW_STACK
>> flag
>> and has a fixed size of min(RLIMIT_STACK, 4GB).
>>
>> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
>> Cc: Kees Cook <keescook@...omium.org>

[...]

>> diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
>> new file mode 100644
>> index 000000000000..5ea2b494e9f9
>> --- /dev/null
>> +++ b/arch/x86/kernel/shstk.c

[...]

>> +int shstk_setup(void)
>> +{
>> +       struct thread_shstk *shstk = &current->thread.shstk;
>> +       unsigned long addr, size;
>> +
>> +       if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
>> +               return -EOPNOTSUPP;
> The only caller of this will skip it if
> !cpu_feature_enabled(X86_FEATURE_SHSTK), so this is dead logic. Same
> pattern in the IBT patch.
> 

Indeed that is the case.  We can simply remove the test of 
X86_FEATURE_SHSTK.

Thanks,
Yu-cheng