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

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Date:   Fri, 12 Mar 2021 08:59:53 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>
Cc:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>, 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 3/12/21 8:55 AM, Jarkko Sakkinen wrote:
>> ENDBR is a special new instruction for the Indirect Branch Tracking
>> (IBT) 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.
> Thank you, this clears the whole thing a lot.
> 
> Doesn't this mean that it could be there just as well unconditionally?

It could be there unconditionally.  But, I think it's still worth the
#ifdef just out of the principle of being as tidy as possible.  The
#ifdef is basically as low cost and low complexity as you get.  It is
also somewhat self-documenting: "This instruction is only necessary when
your CPU supports IBT".