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

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Date:   Thu, 11 Mar 2021 05:36:06 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     "Yu, Yu-cheng" <yu-cheng.yu@...el.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>
Subject: Re: [PATCH v22 8/8] x86/vdso: Add ENDBR64 to __vdso_sgx_enter_enclave

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@...el.com>
> > > Cc: Andy Lutomirski <luto@...nel.org>
> > > Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> > > Cc: Jarkko Sakkinen <jarkko@...nel.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