linux-kernel - Re: [PATCH v40 19/24] x86/vdso: Implement a vDSO for Intel SGX enclave call

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Date:   Sun, 8 Nov 2020 21:20:37 +0100
From:   Jethro Beekman <jethro@...tanix.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>, x86@...nel.org,
        linux-sgx@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Andy Lutomirski <luto@...capital.net>,
        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,
        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, mikko.ylinen@...el.com
Subject: Re: [PATCH v40 19/24] x86/vdso: Implement a vDSO for Intel SGX
 enclave call

On 2020-11-04 15:54, Jarkko Sakkinen wrote:
> From: Sean Christopherson <sean.j.christopherson@...el.com>
> 
> Enclaves encounter exceptions for lots of reasons: everything from enclave
> page faults to NULL pointer dereferences, to system calls that must be
> “proxied” to the kernel from outside the enclave.
> 
> In addition to the code contained inside an enclave, there is also
> supporting code outside the enclave called an “SGX runtime”, which is
> virtually always implemented inside a shared library.  The runtime helps
> build the enclave and handles things like *re*building the enclave if it
> got destroyed by something like a suspend/resume cycle.
> 
> The rebuilding has traditionally been handled in SIGSEGV handlers,
> registered by the library.  But, being process-wide, shared state, signal
> handling and shared libraries do not mix well.
> 
> Introduce a vDSO function call that wraps the enclave entry functions
> (EENTER/ERESUME functions of the ENCLU instruciton) and returns information
> about any exceptions to the caller in the SGX runtime.
> 
> Instead of generating a signal, the kernel places exception information in
> RDI, RSI and RDX. The kernel-provided userspace portion of the vDSO handler
> will place this information in a user-provided buffer or trigger a
> user-provided callback at the time of the exception.
> 
> The vDSO function calling convention uses the standard RDI RSI, RDX, RCX,
> R8 and R9 registers.  This makes it possible to declare the vDSO as a C
> prototype, but other than that there is no specific support for SystemV
> ABI. Things like storing XSAVE are the responsibility of the enclave and
> the runtime.

I suppose this may be covered under "no specific support for SystemV ABI" but with sgx_enclave_run.user_handler=NULL, R12~R15 *will* get clobbered when __vdso_sgx_enter_enclave returns from an SGX AEX. IMO this makes the whole "try to be like System V ABI" rather useless, but I suppose it doesn't matter too much.

--
Jethro Beekman | Fortanix


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