linux-kernel - Re: [PATCH v22 8/8] x86/vdso: Add ENDBR64 to __vdso_sgx_enter

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Date:   Wed, 10 Mar 2021 15:20:20 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>,
        Jarkko Sakkinen <jarkko@...nel.org>
Cc:     x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>,
        Haitao Huang <haitao.huang@...el.com>
Subject: Re: [PATCH v22 8/8] x86/vdso: Add ENDBR64 to __vdso_sgx_enter_enclave

On 3/10/21 2:55 PM, Yu, Yu-cheng wrote:
> On 3/10/2021 2:39 PM, Jarkko Sakkinen wrote:
>> On Wed, Mar 10, 2021 at 02:05:19PM -0800, Yu-cheng Yu wrote:
>>> When CET is enabled, __vdso_sgx_enter_enclave() needs an endbr64
>>> in the beginning of the function.
>>
>> OK.
>>
>> What you should do is to explain what it does and why it's needed.
>>
> 
> The endbr marks a branch target.  Without the "no-track" prefix, if an
> indirect call/jmp reaches a non-endbr opcode, a control-protection fault
> is raised.  Usually endbr's are inserted by the compiler.  For assembly,
> these have to be put in manually.  I will add this in the commit log if
> there is another revision.  Thanks!

This is close, but it's missing a detail or two that I think is
important for someone like Jarkko trying to figure out what it means for
his subsystem or driver.

I'd probably say:

ENDBR is a special new instruction for the Indirect Branch Tracking
(IBR) component of CET.  IBT prevents attacks by ensuring that (most)
indirect branches and function calls may only land at ENDBR
instructions.  Branches that don't follow the rules will result in
control flow (#CF) exceptions.

ENDBR is a noop when IBT is unsupported or disabled.  Most ENDBR
instructions are inserted automatically by the compiler, but branch
targets written in assembly must have ENDBR added manually, like this one.