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

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Date:   Tue, 29 Sep 2020 00:38:11 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>
CC:     "H.J. Lu" <hjl.tools@...il.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 28/09/2020 23:41, Andy Lutomirski wrote:
> On Mon, Sep 28, 2020 at 3:18 PM Dave Hansen <dave.hansen@...el.com> wrote:
>> On 9/28/20 3:06 PM, H.J. Lu wrote:
>>>> I'm open to do either solution. My thinking was to initially do things
>>>> vsgx.S local (i.e. consider ALTERNATIVE post upstreaming) and use the
>>>> above solution but I'm also fine doing ALTERNATIVE. Dave kindly briefed
>>>> on details how that thing works and it should be perfectly usable for
>>>> our use case.
>>>>
>>> Since SHSTK and IBT are enabled per process, not the whole machine,
>>> are you going to patch vDSO on a per-process basis?
>> No.
>>
>> Retpolines mitigate Spectre v2 attacks.  If you're not vulnerable to
>> Spectre v2, you don't need retpolines.
>>
>> All processors which support CET *also* have hardware mitigations
>> against Spectre v2 and don't need retpolines.  Here's all of the
>> possibilities:
>>
>> CET=y, BUG_SPECTRE_V2=y: does not exist
>> CET=n, BUG_SPECTRE_V2=y: vulnerable, use retpoline
>> CET=y, BUG_SPECTRE_V2=n: no retpoline, not vulnerable
>> CET=n, BUG_SPECTRE_V2=n: no retpoline, not vulnerable
> Just to confirm: does this mean that the CPU mitigates against user
> code mistraining the branch predictors for CPL0?

If (and only if) you have eIBRS enabled.

eIBRS should be available on all CET-capable hardware, and Linux ought
to use it by default.

> Because this is the
> vDSO, and the situation we're actually concerned about is user code
> mistraining its own branch predictors.  This could happen
> cross-process or within the same process.

There is nothing (in Intel parts) which prevents mode same-mode training
of indirect branches, either in user or kernel space.

However, an IBPB on context switch should prevent cross-process trailing
attacks.

~Andrew