linux-kernel - Re: [RFC PATCH v1 2/3] x86/vdso: Modify __vdso_sgx_enter_enclave() to allow parameter passing on untrusted stack

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Date:   Tue, 23 Apr 2019 12:44:16 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Cedric Xing <cedric.xing@...el.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, linux-sgx@...r.kernel.org,
        akpm@...ux-foundation.org, Hansen@...ux.intel.com,
        Dave <dave.hansen@...el.com>, Christopherson@...ux.intel.com,
        nhorman@...hat.com, npmccallum@...hat.com, Ayoun@...ux.intel.com,
        Serge <serge.ayoun@...el.com>, Katz-zamir@...ux.intel.com,
        Shay <shay.katz-zamir@...el.com>, Huang@...ux.intel.com,
        Haitao <haitao.huang@...el.com>,
        andriy.shevchenko@...ux.intel.com, tglx@...utronix.de,
        Svahn@...ux.intel.com, Kai <kai.svahn@...el.com>, bp@...en8.de,
        josh@...htriplett.org, luto@...nel.org, Kai <kai.huang@...el.com>,
        rientjes@...gle.com,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Subject: Re: [RFC PATCH v1 2/3] x86/vdso: Modify __vdso_sgx_enter_enclave() to allow parameter passing on untrusted stack



> On Apr 23, 2019, at 12:26 PM, Sean Christopherson <sean.j.christopherson@...el.com> wrote:
> 
>> On Mon, Apr 22, 2019 at 05:37:24PM -0700, Cedric Xing wrote:
>> The previous __vdso_sgx_enter_enclave() requires enclaves to preserve %rsp,
>> which prohibits enclaves from allocating and passing parameters for
>> untrusted function calls (aka. o-calls).
>> 
>> This patch addresses the problem above by introducing a new ABI that preserves
>> %rbp instead of %rsp. Then __vdso_sgx_enter_enclave() can anchor its frame
>> using %rbp so that enclaves are allowed to allocate space on the untrusted
>> stack by decrementing %rsp. Please note that the stack space allocated in such
>> way will be part of __vdso_sgx_enter_enclave()'s frame so will be freed after
>> __vdso_sgx_enter_enclave() returns. Therefore, __vdso_sgx_enter_enclave() has
>> been changed to take a callback function as an optional parameter, which if
>> supplied, will be invoked upon enclave exits (both AEX (Asynchronous Enclave
>> eXit) and normal exits), with the value of %rsp left
>> off by the enclave as a parameter to the callback.
>> 
>> Here's the summary of API/ABI changes in this patch. More details could be
>> found in arch/x86/entry/vdso/vsgx_enter_enclave.S.
>> * 'struct sgx_enclave_exception' is renamed to 'struct sgx_enclave_exinfo'
>>  because it is filled upon both AEX (i.e. exceptions) and normal enclave
>>  exits.
>> * __vdso_sgx_enter_enclave() anchors its frame using %rbp (instead of %rsp in
>>  the previous implementation).
>> * __vdso_sgx_enter_enclave() takes one more parameter - a callback function to
>>  be invoked upon enclave exits. This callback is optional, and if not
>>  supplied, will cause __vdso_sgx_enter_enclave() to return upon enclave exits
>>  (same behavior as previous implementation).
>> * The callback function is given as a parameter the value of %rsp at enclave
>>  exit to address data "pushed" by the enclave. A positive value returned by
>>  the callback will be treated as an ENCLU leaf for re-entering the enclave,
>>  while a zero or negative value will be passed through as the return
>>  value of __vdso_sgx_enter_enclave() to its caller. It's also safe to
>>  leave callback by longjmp() or by throwing a C++ exception.
>> 
>> Signed-off-by: Cedric Xing <cedric.xing@...el.com>
>> ---
>> arch/x86/entry/vdso/vsgx_enter_enclave.S | 156 ++++++++++++++---------
>> arch/x86/include/uapi/asm/sgx.h          |  14 +-
>> 2 files changed, 100 insertions(+), 70 deletions(-)
>> 
>> diff --git a/arch/x86/entry/vdso/vsgx_enter_enclave.S b/arch/x86/entry/vdso/vsgx_enter_enclave.S
>> index fe0bf6671d6d..210f4366374a 100644
>> --- a/arch/x86/entry/vdso/vsgx_enter_enclave.S
>> +++ b/arch/x86/entry/vdso/vsgx_enter_enclave.S
>> @@ -14,88 +14,118 @@
>> .code64
>> .section .text, "ax"
>> 
>> -#ifdef SGX_KERNEL_DOC
> 
> This #ifdef and the pseudo-C code below has a functional purpose.  From
> the original commit:
> 
>  Note, the C-like pseudocode describing the assembly routine is wrapped
>  in a non-existent macro instead of in a comment to trick kernel-doc into
>  auto-parsing the documentation and function prototype.  This is a double
>  win as the pseudocode is intended to aid kernel developers, not userland
>  enclave developers.
> 
> We don't need full pseudocode, but a C-like prototype is necessary to get
> the kernel-doc comment parsed correctly.

That should be explained in a comment :)

—Andy