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Date:   Mon, 28 Sep 2020 03:58:42 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
To:     Andrew Cooper <andrew.cooper3@citrix.com>
Cc:     x86@kernel.org, linux-sgx@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Jethro Beekman <jethro@fortanix.com>,
        Cedric Xing <cedric.xing@intel.com>, akpm@linux-foundation.org,
        andriy.shevchenko@linux.intel.com, asapek@google.com, bp@alien8.de,
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        cyhanish@google.com, dave.hansen@intel.com, haitao.huang@intel.com,
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        rientjes@google.com, tglx@linutronix.de, yaozhangx@google.com,
        "H.J. Lu" <hjl.tools@gmail.com>,
        Yu-cheng Yu <yu-cheng.yu@intel.com>
Subject: Re: [PATCH v38 21/24] x86/vdso: Implement a vDSO for Intel SGX
 enclave call
Message-ID: <20200928005842.GC6704@linux.intel.com>
References: <20200915112842.897265-1-jarkko.sakkinen@linux.intel.com>
 <20200915112842.897265-22-jarkko.sakkinen@linux.intel.com>
 <721ca14e-21df-3df1-7bef-0b00d0ff90c3@citrix.com>
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Precedence: bulk

On Fri, Sep 25, 2020 at 07:23:59PM +0100, Andrew Cooper wrote:
> On 15/09/2020 12:28, Jarkko Sakkinen wrote:
> > diff --git a/arch/x86/entry/vdso/vsgx_enter_enclave.S b/arch/x86/entry/vdso/vsgx_enter_enclave.S
> > new file mode 100644
> > index 000000000000..adbd59d41517
> > --- /dev/null
> > +++ b/arch/x86/entry/vdso/vsgx_enter_enclave.S
> > @@ -0,0 +1,157 @@
> > +SYM_FUNC_START(__vdso_sgx_enter_enclave)
> > <snip>
> > +.Lretpoline:
> > +	call	2f
> > +1:	pause
> > +	lfence
> > +	jmp	1b
> > +2:	mov	%rax, (%rsp)
> > +	ret
> 
> I hate to throw further spanners in the work, but this is not compatible
> with CET, and the user shadow stack work in progress.

CET goes beyond my expertise. Can you describe, at least rudimentary,
how this code is not compatible?

I know CET only conceptual level (separate stack holding return
addresses as an measure against return oriented programming (ROP)).

> Whichever of these two large series lands first is going to inflict
> fixing this problem on the other.
> 
> As the vdso text is global (to a first approximation), it must not be a
> retpoline if any other process is liable to want to use CET-SS.

Why is that?

> If the retpoline really does need to stay, then the vdso probably needs
> to gain suitable __x86_indirect_thunk_%reg thunks which are patched at
> boot based on the system properties.
> 
> ~Andrew

aka without CET it is patched? With CET, not?

/Jarkko