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Subject: Re: [RFC PATCH v1 2/3] x86/vdso: Modify __vdso_sgx_enter_enclave() to allow parameter passing on untrusted stack
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20190423192654.GA12691@linux.intel.com>
Date:   Tue, 23 Apr 2019 12:44:16 -0700
Cc:     Cedric Xing <cedric.xing@intel.com>, linux-kernel@vger.kernel.org,
        x86@kernel.org, linux-sgx@vger.kernel.org,
        akpm@linux-foundation.org, Hansen@linux.intel.com,
        Dave <dave.hansen@intel.com>, Christopherson@linux.intel.com,
        nhorman@redhat.com, npmccallum@redhat.com, Ayoun@linux.intel.com,
        Serge <serge.ayoun@intel.com>, Katz-zamir@linux.intel.com,
        Shay <shay.katz-zamir@intel.com>, Huang@linux.intel.com,
        Haitao <haitao.huang@intel.com>,
        andriy.shevchenko@linux.intel.com, tglx@linutronix.de,
        Svahn@linux.intel.com, Kai <kai.svahn@intel.com>, bp@alien8.de,
        josh@joshtriplett.org, luto@kernel.org, Kai <kai.huang@intel.com>,
        rientjes@google.com,
        Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
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Message-Id: <B84F5867-A729-46E2-88F2-7D6F7C4DEE79@amacapital.net>
References: <20190417103938.7762-1-jarkko.sakkinen@linux.intel.com> <cover.1555965327.git.cedric.xing@intel.com> <ba2a51568f3adaf74994d33ea3cbee570e20c6f6.1555965327.git.cedric.xing@intel.com> <20190423192654.GA12691@linux.intel.com>
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Apr 23, 2019, at 12:26 PM, Sean Christopherson <sean.j.christopherson@i=
ntel.com> wrote:
>=20
>> On Mon, Apr 22, 2019 at 05:37:24PM -0700, Cedric Xing wrote:
>> The previous __vdso_sgx_enter_enclave() requires enclaves to preserve %rs=
p,
>> which prohibits enclaves from allocating and passing parameters for
>> untrusted function calls (aka. o-calls).
>>=20
>> This patch addresses the problem above by introducing a new ABI that pres=
erves
>> %rbp instead of %rsp. Then __vdso_sgx_enter_enclave() can anchor its fram=
e
>> using %rbp so that enclaves are allowed to allocate space on the untruste=
d
>> 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 a=
fter
>> __vdso_sgx_enter_enclave() returns. Therefore, __vdso_sgx_enter_enclave()=
 has
>> been changed to take a callback function as an optional parameter, which i=
f
>> supplied, will be invoked upon enclave exits (both AEX (Asynchronous Encl=
ave
>> eXit) and normal exits), with the value of %rsp left
>> off by the enclave as a parameter to the callback.
>>=20
>> Here's the summary of API/ABI changes in this patch. More details could b=
e
>> 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 %rs=
p in
>>  the previous implementation).
>> * __vdso_sgx_enter_enclave() takes one more parameter - a callback functi=
on to
>>  be invoked upon enclave exits. This callback is optional, and if not
>>  supplied, will cause __vdso_sgx_enter_enclave() to return upon enclave e=
xits
>>  (same behavior as previous implementation).
>> * The callback function is given as a parameter the value of %rsp at encl=
ave
>>  exit to address data "pushed" by the enclave. A positive value returned b=
y
>>  the callback will be treated as an ENCLU leaf for re-entering the enclav=
e,
>>  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.
>>=20
>> Signed-off-by: Cedric Xing <cedric.xing@intel.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(-)
>>=20
>> diff --git a/arch/x86/entry/vdso/vsgx_enter_enclave.S b/arch/x86/entry/vd=
so/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"
>>=20
>> -#ifdef SGX_KERNEL_DOC
>=20
> This #ifdef and the pseudo-C code below has a functional purpose.  From
> the original commit:
>=20
>  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.
>=20
> 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 :)

=E2=80=94Andy