linux-kernel - Re: [PATCH v38 21/24] x86/vdso: Implement a vDSO for Intel SGX enclave call

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Date:   Mon, 28 Sep 2020 11:17:42 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andy Lutomirski <luto@...nel.org>, "H.J. Lu" <hjl.tools@...il.com>
Cc:     Andrew Cooper <andrew.cooper3@...rix.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        linux-sgx@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Jethro Beekman <jethro@...tanix.com>,
        Cedric Xing <cedric.xing@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        asapek@...gle.com, Borislav Petkov <bp@...en8.de>,
        chenalexchen@...gle.com, Conrad Parker <conradparker@...gle.com>,
        cyhanish@...gle.com, "Huang, Haitao" <haitao.huang@...el.com>,
        Josh Triplett <josh@...htriplett.org>,
        "Huang, Kai" <kai.huang@...el.com>,
        "Svahn, Kai" <kai.svahn@...el.com>, Keith Moyer <kmoy@...gle.com>,
        Christian Ludloff <ludloff@...gle.com>,
        Neil Horman <nhorman@...hat.com>,
        Nathaniel McCallum <npmccallum@...hat.com>,
        Patrick Uiterwijk <puiterwijk@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>, yaozhangx@...gle.com,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v38 21/24] x86/vdso: Implement a vDSO for Intel SGX
 enclave call

On 9/28/20 11:12 AM, Andy Lutomirski wrote:
>> endbr64
>> /* Check if shadow stack is in use.  NB: R11 is the only usable
>>    scratch register for function calls.  */
>> xorl %r11d, %r11d
>> rdsspq %r11
>> testq %r11, %r11
>> jnz 3f
>> call 2f
>> 1:
>> pause
>> lfence
>> jmp 1b
>> 2:
>> mov %rax, (%rsp)
>> ret
>> 3:
>> /* Shadow stack is in use.  Make the indirect call.  */
>> call *%rax
>> ret
> What do we expect user programs to do on CET systems?  It would be
> nice if we could instead ALTERNATIVE this out if X86_FEATURE_SHSTK.

Shouldn't we just be able to use X86_FEATURE_RETPOLINE?

We probably need a mechanism to force X86_FEATURE_SHSTK and
X86_FEATURE_RETPOLINE to be mutually exclusive if we don't have one already.