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

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Message-ID: <20200928220740.GG2705@linux.intel.com>
Date:   Tue, 29 Sep 2020 01:07:40 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>, "H.J. Lu" <hjl.tools@...il.com>,
        Andrew Cooper <andrew.cooper3@...rix.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 Mon, Sep 28, 2020 at 11:17:42AM -0700, Dave Hansen wrote:
> 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.

First of all: lets go with boot time patching instead of dynamic
detection. It's both easier to implement and by all other merits makes a
lot more sense. It was just a thing that I've not used before.

That sorted out, does it matter which direction I look it at? I could
use either feature flag as basis (and I do not have a personal
preference here).

/Jarkko