Re: [PATCH v22 8/8] x86/vdso: Add ENDBR64 to __vdso_sgx_enter_enclave

From: Jarkko Sakkinen <jarkko@kernel.org>
To: "Yu, Yu-cheng" <yu-cheng.yu@intel.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>
Subject: Re: [PATCH v22 8/8] x86/vdso: Add ENDBR64 to __vdso_sgx_enter_enclave
Date: Thu, 11 Mar 2021 05:36:06 +0200	[thread overview]
Message-ID: <YEmQJjwjs8UCEO2F@kernel.org> (raw)
In-Reply-To: <8b8efe44-b79f-ce29-ee28-066f88c93840@intel.com>

On Wed, Mar 10, 2021 at 02:55:55PM -0800, 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!

Thanks for the explanation. There is another revision, because this is
lacking from the commit message.

Does it do any harm to put it there unconditionally?

> 
> --
> Yu-cheng
> 
> > > 
> > > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> > > Cc: Andy Lutomirski <luto@kernel.org>
> > > Cc: Dave Hansen <dave.hansen@linux.intel.com>
> > > Cc: Jarkko Sakkinen <jarkko@kernel.org>
> > > ---
> > >   arch/x86/entry/vdso/vsgx.S | 3 +++
> > >   1 file changed, 3 insertions(+)
> > > 
> > > diff --git a/arch/x86/entry/vdso/vsgx.S b/arch/x86/entry/vdso/vsgx.S
> > > index 86a0e94f68df..a70d4d09f713 100644
> > > --- a/arch/x86/entry/vdso/vsgx.S
> > > +++ b/arch/x86/entry/vdso/vsgx.S
> > > @@ -27,6 +27,9 @@
> > >   SYM_FUNC_START(__vdso_sgx_enter_enclave)
> > >   	/* Prolog */
> > >   	.cfi_startproc
> > > +#ifdef CONFIG_X86_CET
> > > +	endbr64
> > > +#endif
> > >   	push	%rbp
> > >   	.cfi_adjust_cfa_offset	8
> > >   	.cfi_rel_offset		%rbp, 0
> > > -- 
> > > 2.21.0
> > > 
> > > 
> > 
> > /Jarkko
> > 
> 
> 

/Jarkko