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Date:   Mon, 28 Sep 2020 23:42:58 +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,
        chenalexchen@google.com, conradparker@google.com,
        cyhanish@google.com, dave.hansen@intel.com, haitao.huang@intel.com,
        josh@joshtriplett.org, kai.huang@intel.com, kai.svahn@intel.com,
        kmoy@google.com, ludloff@google.com, luto@kernel.org,
        nhorman@redhat.com, npmccallum@redhat.com, puiterwijk@redhat.com,
        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: <20200928204258.GA2705@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>
 <20200928005842.GC6704@linux.intel.com>
 <85bc15d5-93cd-e332-ae9a-1e1e66e1181d@citrix.com>
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Precedence: bulk

On Mon, Sep 28, 2020 at 05:44:35PM +0100, Andrew Cooper wrote:
> On 28/09/2020 01:58, Jarkko Sakkinen wrote:
> > 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?
> 
> CET Shadow Stacks detect attacks which modify the return address on the
> stack.
> 
> Retpoline *is* a ROP gadget.? It really does modify the return address
> on the stack, even if its purpose is defensive (vs Spectre v2) rather
> than malicious.

Aah. I get that, yes.

Kernel is full of retpoline but I presume that ring-0 does not use CET.

The situation with callback is follows: for a run-time the user_handler
by all practical means is always the same. There is ever only one user
handler that gets executed. I.e. the indirect callback will always lead
to the same thing. I wonder how much assets an adversary would get if
we just remove retpoline bits (not much thinking done yet on that).

/Jarkko