linux-kernel - Re: [PATCH v38 21/24] x86/vdso: Implement a vDSO for Intel SGX enclave call

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Date:   Mon, 28 Sep 2020 17:44:35 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
CC:     <x86@...nel.org>, <linux-sgx@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Andy Lutomirski <luto@...capital.net>,
        Jethro Beekman <jethro@...tanix.com>,
        Cedric Xing <cedric.xing@...el.com>,
        <akpm@...ux-foundation.org>, <andriy.shevchenko@...ux.intel.com>,
        <asapek@...gle.com>, <bp@...en8.de>, <chenalexchen@...gle.com>,
        <conradparker@...gle.com>, <cyhanish@...gle.com>,
        <dave.hansen@...el.com>, <haitao.huang@...el.com>,
        <josh@...htriplett.org>, <kai.huang@...el.com>,
        <kai.svahn@...el.com>, <kmoy@...gle.com>, <ludloff@...gle.com>,
        <luto@...nel.org>, <nhorman@...hat.com>, <npmccallum@...hat.com>,
        <puiterwijk@...hat.com>, <rientjes@...gle.com>,
        <tglx@...utronix.de>, <yaozhangx@...gle.com>,
        "H.J. Lu" <hjl.tools@...il.com>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [PATCH v38 21/24] x86/vdso: Implement a vDSO for Intel SGX
 enclave call

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.

~Andrew