linux-kernel - Re: [PATCH v24 9/9] x86/vdso: Add ENDBR to __vdso_sgx_enter

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Message-ID: <8bc0bfb1-27f4-541e-f80c-97d16d5733bb@intel.com>
Date:   Mon, 24 May 2021 12:01:29 -0700
From:   "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
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>,
        Jarkko Sakkinen <jarkko@...nel.org>
Subject: Re: [PATCH v24 9/9] x86/vdso: Add ENDBR to __vdso_sgx_enter_enclave

On 5/24/2021 11:51 AM, Sean Christopherson wrote:
> On Thu, Apr 01, 2021, Yu-cheng Yu 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.
>>
>> Add ENDBR to __vdso_sgx_enter_enclave() branch targets.
>                                            ^
>                                            |- indirect
> 
> After reading the changelog, I was expecting ENDBR on every label.
> 

Sorry about the confusion.

>> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
>> Cc: Andy Lutomirski <luto@...nel.org>
>> Cc: Borislav Petkov <bp@...en8.de>
>> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
>> Cc: Jarkko Sakkinen <jarkko@...nel.org>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> ---
>>   arch/x86/entry/vdso/vsgx.S | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/arch/x86/entry/vdso/vsgx.S b/arch/x86/entry/vdso/vsgx.S
>> index 86a0e94f68df..c63eafa54abd 100644
>> --- a/arch/x86/entry/vdso/vsgx.S
>> +++ b/arch/x86/entry/vdso/vsgx.S
>> @@ -4,6 +4,7 @@
>>   #include <asm/export.h>
>>   #include <asm/errno.h>
>>   #include <asm/enclu.h>
>> +#include <asm/vdso.h>
>>   
>>   #include "extable.h"
>>   
>> @@ -27,6 +28,7 @@
>>   SYM_FUNC_START(__vdso_sgx_enter_enclave)
>>   	/* Prolog */
>>   	.cfi_startproc
>> +	ENDBR
>>   	push	%rbp
>>   	.cfi_adjust_cfa_offset	8
>>   	.cfi_rel_offset		%rbp, 0
>> @@ -62,6 +64,7 @@ SYM_FUNC_START(__vdso_sgx_enter_enclave)
>>   .Lasync_exit_pointer:
>>   .Lenclu_eenter_eresume:
>>   	enclu
>> +	ENDBR
> 
> It would be better to move this below the comment about EEXIT.  As is, it looks
> like a misplaced annotation on the AEP.  The AEP doesn't need ENDBR, it's the
> EEXIT target that needs ENDBR because EEXIT is treated as an indirect branch.
> 
> Might also be helpful for future readers to explicitly state in the changelog
> that EEXIT is considered an indirect branch.
> 
> I.e.
> 
>>   	/* EEXIT jumps here unless the enclave is doing something fancy. */
> 	ENDBR
>>   	mov	SGX_ENCLAVE_OFFSET_OF_RUN(%rbp), %rbx
>> @@ -91,6 +94,7 @@ SYM_FUNC_START(__vdso_sgx_enter_enclave)
>>   	jmp	.Lout
>>   
>>   .Lhandle_exception:
>> +	ENDBR
>>   	mov	SGX_ENCLAVE_OFFSET_OF_RUN(%rbp), %rbx
>>   
>>   	/* Set the exception info. */

Thanks, I will fix that.

Yu-cheng