Re: [PATCH v24 9/9] x86/vdso: Add ENDBR to __vdso_sgx_enter_enclave - Yu, Yu-cheng

From: "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
To: Sean Christopherson <seanjc@google.com>
Cc: x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
	linux-mm@kvack.org, linux-arch@vger.kernel.org,
	linux-api@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
	Andy Lutomirski <luto@kernel.org>,
	Balbir Singh <bsingharora@gmail.com>,
	Borislav Petkov <bp@alien8.de>,
	Cyrill Gorcunov <gorcunov@gmail.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Eugene Syromiatnikov <esyr@redhat.com>,
	Florian Weimer <fweimer@redhat.com>,
	"H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Kees Cook <keescook@chromium.org>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Nadav Amit <nadav.amit@gmail.com>,
	Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
	Peter Zijlstra <peterz@infradead.org>,
	Randy Dunlap <rdunlap@infradead.org>,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
	Dave Martin <Dave.Martin@arm.com>,
	Weijiang Yang <weijiang.yang@intel.com>,
	Pengfei Xu <pengfei.xu@intel.com>,
	Haitao Huang <haitao.huang@intel.com>,
	Jarkko Sakkinen <jarkko@kernel.org>
Subject: Re: [PATCH v24 9/9] x86/vdso: Add ENDBR to __vdso_sgx_enter_enclave
Date: Mon, 24 May 2021 12:01:29 -0700	[thread overview]
Message-ID: <8bc0bfb1-27f4-541e-f80c-97d16d5733bb@intel.com> (raw)
In-Reply-To: <YKv1o9XCMrLzZ5xW@google.com>

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@intel.com>
>> Cc: Andy Lutomirski <luto@kernel.org>
>> Cc: Borislav Petkov <bp@alien8.de>
>> Cc: Dave Hansen <dave.hansen@linux.intel.com>
>> Cc: Jarkko Sakkinen <jarkko@kernel.org>
>> Cc: Peter Zijlstra <peterz@infradead.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