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Date:   Tue, 29 Sep 2020 00:41:05 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
To:     "H.J. Lu" <hjl.tools@gmail.com>
Cc:     Andrew Cooper <andrew.cooper3@citrix.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        linux-sgx@vger.kernel.org, LKML <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>,
        Andrew Morton <akpm@linux-foundation.org>,
        andriy.shevchenko@linux.intel.com, asapek@google.com,
        Borislav Petkov <bp@alien8.de>, chenalexchen@google.com,
        conradparker@google.com, cyhanish@google.com,
        Dave Hansen <dave.hansen@intel.com>,
        "Huang, Haitao" <haitao.huang@intel.com>, josh@joshtriplett.org,
        kai.huang@intel.com, kai.svahn@intel.com, kmoy@google.com,
        Christian Ludloff <ludloff@google.com>,
        Andy Lutomirski <luto@kernel.org>, nhorman@redhat.com,
        npmccallum@redhat.com, puiterwijk@redhat.com, rientjes@google.com,
        Thomas Gleixner <tglx@linutronix.de>, yaozhangx@google.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: <20200928214105.GD2705@linux.intel.com>
References: <20200915112842.897265-1-jarkko.sakkinen@linux.intel.com>
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 <85bc15d5-93cd-e332-ae9a-1e1e66e1181d@citrix.com>
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Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo
Precedence: bulk

On Mon, Sep 28, 2020 at 11:07:47AM -0700, H.J. Lu wrote:
> On Mon, Sep 28, 2020 at 9:44 AM Andrew Cooper <andrew.cooper3@citrix.com> 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.
> >
> > >> 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?
> >
> > Because when CET-SS is enabled, the ret will suffer a #CP exception
> > (return address on the stack not matching the one recorded in the shadow
> > stack), which I presume/hope is wired into SIGSEGV.
> >
> 
> Here is the CET compatible retpoline:
> 
> endbr64
> /* Check if shadow stack is in use.  NB: R11 is the only usable
>    scratch register for function calls.  */
> xorl %r11d, %r11d
> rdsspq %r11
> testq %r11, %r11
> jnz 3f
> call 2f
> 1:
> pause
> lfence
> jmp 1b
> 2:
> mov %rax, (%rsp)
> ret
> 3:
> /* Shadow stack is in use.  Make the indirect call.  */
> call *%rax
> ret

Right, so I have actually two alternatives: this and boot time patching:

https://lkml.org/lkml/2020/9/25/1122

/Jarkko