linux-kernel - Re: [PATCH v23 6/9] x86/entry: Introduce ENDBR macro

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Date:   Tue, 16 Mar 2021 10:12:39 -0700
From:   "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To:     Dave Hansen <dave.hansen@...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>
Cc:     Jarkko Sakkinen <jarkko@...nel.org>
Subject: Re: [PATCH v23 6/9] x86/entry: Introduce ENDBR macro

On 3/16/2021 8:49 AM, Dave Hansen wrote:
> On 3/16/21 8:13 AM, Yu-cheng Yu wrote:
>> --- a/arch/x86/entry/calling.h
>> +++ b/arch/x86/entry/calling.h
>> @@ -392,3 +392,21 @@ For 32-bit we have the following conventions - kernel is built with
>>   .endm
>>   
>>   #endif /* CONFIG_SMP */
>> +/*
>> + * ENDBR is an instruction for the Indirect Branch Tracking (IBT) component
>> + * of CET.  IBT prevents attacks by ensuring that (most) indirect branches
>> + * 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.
>> + */
>> +.macro ENDBR
>> +#ifdef CONFIG_X86_CET
>> +#ifdef __i386__
>> +	endbr32
>> +#else
>> +	endbr64
>> +#endif
>> +#endif
>> +.endm
> 
> Is "#ifdef __i386__" the right thing to use here?  I guess ENDBR only
> ends up getting used in the VDSO, but there's a lot of
> non-userspace-exposed stuff in calling.h.  It seems a bit weird to have
> the normally userspace-only __i386__ in there.
> 
> I don't see any existing direct use of __i386__ in arch/x86/entry/vdso.
> 

Good point.  My thought was, __i386__ comes from the compiler having the 
-m32 command-line option, and it is not dependent on anything else.

Alternatively, there is another compiler-defined macro _CET_ENDBR that 
can be used.  We can put the following in calling.h:

#ifdef __CET__
#include <cet.h>
#else
#define _CET_ENDBR
#endif

and then use _CET_ENDBR in other files.  How is that?

In the future, in case we have kernel-mode IBT, ENDBR macros are also 
needed for other assembly files.

Thanks,
Yu-cheng