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From: Nathaniel McCallum <npmccallum@redhat.com>
To: "Xing, Cedric" <cedric.xing@intel.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>,
	Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
	linux-kernel@vger.kernel.org, x86@kernel.org,
	linux-sgx@vger.kernel.org, akpm@linux-foundation.org,
	dave.hansen@intel.com, Neil Horman <nhorman@redhat.com>,
	"Huang, Haitao" <haitao.huang@intel.com>,
	andriy.shevchenko@linux.intel.com, tglx@linutronix.de, "Svahn,
	Kai" <kai.svahn@intel.com>,
	bp@alien8.de, Josh Triplett <josh@joshtriplett.org>,
	luto@kernel.org, kai.huang@intel.com,
	David Rientjes <rientjes@google.com>,
	Patrick Uiterwijk <puiterwijk@redhat.com>,
	Andy Lutomirski <luto@amacapital.net>,
	Jethro Beekman <jethro@fortanix.com>,
	Connor Kuehl <ckuehl@redhat.com>,
	Harald Hoyer <harald@redhat.com>,
	Lily Sturmann <lsturman@redhat.com>
Subject: Re: [PATCH v28 21/22] x86/vdso: Implement a vDSO for Intel SGX enclave call
Date: Fri, 20 Mar 2020 11:53:45 -0400	[thread overview]
Message-ID: <CAOASepM4uWH=shAj17j9E=Q55GcWQg3T6p9b88XZSn2tzx2Tmw@mail.gmail.com> (raw)
In-Reply-To: <CAOASepPZ727dKLNaus2wGY3CLNE1SNi-5QyvS5OMxcCYbN5Rdw@mail.gmail.com>

On Wed, Mar 18, 2020 at 9:01 AM Nathaniel McCallum
<npmccallum@redhat.com> wrote:
>
> On Tue, Mar 17, 2020 at 6:23 PM Xing, Cedric <cedric.xing@intel.com> wrote:
> >
> > On 3/17/2020 9:50 AM, Nathaniel McCallum wrote:
> > > On Mon, Mar 16, 2020 at 8:18 PM Xing, Cedric <cedric.xing@intel.com> wrote:
> > >>
> > >> On 3/16/2020 4:59 PM, Sean Christopherson wrote:
> > >>> On Mon, Mar 16, 2020 at 04:50:26PM -0700, Xing, Cedric wrote:
> > >>>> On 3/16/2020 3:53 PM, Sean Christopherson wrote:
> > >>>>> On Mon, Mar 16, 2020 at 11:38:24PM +0200, Jarkko Sakkinen wrote:
> > >>>>>>> My suggestions explicitly maintained robustness, and in fact increased
> > >>>>>>> it. If you think we've lost capability, please speak with specificity
> > >>>>>>> rather than in vague generalities. Under my suggestions we can:
> > >>>>>>> 1. call the vDSO from C
> > >>>>>>> 2. pass context to the handler
> > >>>>>>> 3. have additional stack manipulation options in the handler
> > >>>>>>>
> > >>>>>>> The cost for this is a net 2 additional instructions. No existing
> > >>>>>>> capability is lost.
> > >>>>>>
> > >>>>>> My vague generality in this case is just that the whole design
> > >>>>>> approach so far has been to minimize the amount of wrapping to
> > >>>>>> EENTER.
> > >>>>>
> > >>>>> Yes and no.   If we wanted to minimize the amount of wrapping around the
> > >>>>> vDSO's ENCLU then we wouldn't have the exit handler shenanigans in the
> > >>>>> first place.  The whole process has been about balancing the wants of each
> > >>>>> use case against the overall quality of the API and code.
> > >>>>>
> > >>>> The design of this vDSO API was NOT to minimize wrapping, but to allow
> > >>>> maximal flexibility. More specifically, we strove not to restrict how info
> > >>>> was exchanged between the enclave and its host process. After all, calling
> > >>>> convention is compiler specific - i.e. the enclave could be built by a
> > >>>> different compiler (e.g. MSVC) that doesn't share the same list of CSRs as
> > >>>> the host process. Therefore, the API has been implemented to pass through
> > >>>> virtually all registers except those used by EENTER itself. Similarly, all
> > >>>> registers are passed back from enclave to the caller (or the exit handler)
> > >>>> except those used by EEXIT. %rbp is an exception because the vDSO API has to
> > >>>> anchor the stack, using either %rsp or %rbp. We picked %rbp to allow the
> > >>>> enclave to allocate space on the stack.
> > >>>
> > >>> And unless I'm missing something, using %rcx to pass @leaf would still
> > >>> satisfy the above, correct?  Ditto for saving/restoring %rbx.
> > >>>
> > >>> I.e. a runtime that's designed to work with enclave's using a different
> > >>> calling convention wouldn't be able to take advantage of being able to call
> > >>> the vDSO from C, but neither would it take on any meaningful burden.
> > >>>
> > >> Not exactly.
> > >>
> > >> If called directly from C code, the caller would expect CSRs to be
> > >> preserved.
> > >
> > > Correct. This requires collaboration between the caller of the vDSO
> > > and the enclave.
> > >
> > >> Then who should preserve CSRs?
> > >
> > > The enclave.
> > >
> > >> It can't be the enclave
> > >> because it may not follow the same calling convention.
> > >
> > > This is incorrect. You are presuming there is not tight integration
> > > between the caller of the vDSO and the enclave. In my case, the
> > > integration is total and complete. We have working code today that
> > > does this.
> > >
> > >> Moreover, the
> > >> enclave may run into an exception, in which case it doesn't have the
> > >> ability to restore CSRs.
> > >
> > > There are two solutions to this:
> > > 1. Write the handler in assembly and don't return to C on AEX.
> > > 2. The caller can simply preserve the registers. Nothing stops that.
> > >
> > > We have implemented #1.
> > >
> > What if the enclave cannot proceed due to an unhandled exception so the
> > execution has to get back to the C caller of the vDSO API?
>
> mov $60, %rax
> mov $1, %rdi
> syscall
>
> We exit in all such cases.

Another solution is for the enclave to push the non-volatile registers
to the non-enclave stack upon entry and let the handler restore them.
That works for both EEXIT and AEX and you can return to C code then.

> > It seems to me the caller has to preserve CSRs by itself, otherwise it
> > cannot continue execution after any enclave exception. Passing @leaf in
> > %ecx will allow saving/restoring CSRs in C by setjmp()/longjmp(), with
> > the help of an exit handler. But if the C caller has already preserved
> > CSRs, why preserve CSRs again inside the enclave? It looks to me things
> > can be simplified only if the host process handles no enclave exceptions
> > (or exceptions inside the enclave will crash the calling thread). Thus
> > the only case of enclave EEXIT'ing back to its caller is considered
> > valid, hence the enclave will always be able to restore CSRs, so that
> > neither vDSO nor its caller has to preserve CSRs.
> >
> > Is my understanding correct?
> >


  reply	other threads:[~2020-03-20 15:54 UTC|newest]

Thread overview: 110+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-03 23:35 [PATCH v28 00/22] Intel SGX foundations Jarkko Sakkinen
2020-03-03 23:35 ` [PATCH v28 01/22] x86/sgx: Update MAINTAINERS Jarkko Sakkinen
2020-03-03 23:35 ` [PATCH v28 02/22] x86/cpufeatures: x86/msr: Add Intel SGX hardware bits Jarkko Sakkinen
2020-03-03 23:35 ` [PATCH v28 03/22] x86/cpufeatures: x86/msr: Intel SGX Launch Control " Jarkko Sakkinen
2020-03-03 23:35 ` [PATCH v28 04/22] x86/mm: x86/sgx: Signal SIGSEGV with PF_SGX Jarkko Sakkinen
2020-03-03 23:35 ` [PATCH v28 05/22] x86/sgx: Add SGX microarchitectural data structures Jarkko Sakkinen
2020-03-03 23:35 ` [PATCH v28 06/22] x86/sgx: Add wrappers for ENCLS leaf functions Jarkko Sakkinen
2020-03-09 21:14   ` Sean Christopherson
2020-03-03 23:35 ` [PATCH v28 07/22] x86/cpu/intel: Detect SGX supprt Jarkko Sakkinen
2020-03-09 21:56   ` Sean Christopherson
2020-03-11 17:03     ` Jarkko Sakkinen
2020-03-03 23:35 ` [PATCH v28 08/22] x86/sgx: Enumerate and track EPC sections Jarkko Sakkinen
2020-03-03 23:35 ` [PATCH v28 09/22] x86/sgx: Add functions to allocate and free EPC pages Jarkko Sakkinen
2020-03-03 23:35 ` [PATCH v28 10/22] mm: Introduce vm_ops->may_mprotect() Jarkko Sakkinen
2020-03-03 23:35 ` [PATCH v28 11/22] x86/sgx: Linux Enclave Driver Jarkko Sakkinen
2020-03-05 17:40   ` Sean Christopherson
2020-03-05 18:24     ` Jethro Beekman
2020-03-05 19:04       ` Sean Christopherson
2020-03-06 19:00       ` Jarkko Sakkinen
2020-03-19 18:22         ` Dr. Greg
2020-03-06 18:58     ` Jarkko Sakkinen
2020-03-03 23:35 ` [PATCH v28 12/22] docs: x86/sgx: Document SGX micro architecture and kernel internals Jarkko Sakkinen
2020-03-03 23:36 ` [PATCH v28 13/22] selftests/x86: Recurse into subdirectories Jarkko Sakkinen
2020-03-03 23:36 ` [PATCH v28 14/22] selftests/x86: Add a selftest for SGX Jarkko Sakkinen
2020-03-04 19:27   ` Nathaniel McCallum
2020-03-05 11:33     ` Jarkko Sakkinen
2020-03-06 15:42       ` Dr. Greg
2020-03-06 19:07         ` Jarkko Sakkinen
2020-03-07 17:42           ` Dr. Greg
2020-03-10 13:08             ` Jarkko Sakkinen
2020-03-11 13:28       ` Jarkko Sakkinen
2020-03-11 16:40         ` Sean Christopherson
2020-03-13 19:24           ` Jarkko Sakkinen
2020-03-04 19:44   ` Nathaniel McCallum
2020-03-04 19:51   ` Nathaniel McCallum
2020-03-06  5:32   ` Dr. Greg
2020-03-06 19:04     ` Jarkko Sakkinen
2020-03-10 19:29     ` Haitao Huang
2020-03-11  9:13       ` Dr. Greg
2020-03-11 17:15         ` Haitao Huang
2020-03-17  1:07       ` Dr. Greg
2020-03-03 23:36 ` [PATCH v28 15/22] x86/sgx: Add provisioning Jarkko Sakkinen
2020-03-03 23:36 ` [PATCH v28 16/22] x86/sgx: Add a page reclaimer Jarkko Sakkinen
2020-03-05 19:03   ` Sean Christopherson
2020-03-06 18:47     ` Jarkko Sakkinen
2020-03-12 18:38       ` Sean Christopherson
2020-03-15  0:27         ` Jarkko Sakkinen
2020-03-15  1:17           ` Jarkko Sakkinen
2020-03-09 21:16   ` Sean Christopherson
2020-03-03 23:36 ` [PATCH v28 17/22] x86/sgx: ptrace() support for the SGX driver Jarkko Sakkinen
2020-03-03 23:36 ` [PATCH v28 18/22] x86/vdso: Add support for exception fixup in vDSO functions Jarkko Sakkinen
2020-03-03 23:36 ` [PATCH v28 19/22] x86/fault: Add helper function to sanitize error code Jarkko Sakkinen
2020-03-03 23:36 ` [PATCH v28 20/22] x86/traps: Attempt to fixup exceptions in vDSO before signaling Jarkko Sakkinen
2020-03-03 23:36 ` [PATCH v28 21/22] x86/vdso: Implement a vDSO for Intel SGX enclave call Jarkko Sakkinen
2020-03-11 17:30   ` Nathaniel McCallum
2020-03-11 17:38     ` Jethro Beekman
2020-03-11 19:15       ` Nathaniel McCallum
2020-03-13 15:48       ` Nathaniel McCallum
2020-03-13 16:46         ` Sean Christopherson
2020-03-13 18:32           ` Nathaniel McCallum
2020-03-13 18:44             ` Sean Christopherson
2020-03-13 20:14               ` Nathaniel McCallum
2020-03-13 22:08                 ` Sean Christopherson
2020-03-14 14:10                   ` Nathaniel McCallum
2020-03-18 23:40                     ` Sean Christopherson
2020-03-19  0:38                       ` Xing, Cedric
2020-03-19  1:03                         ` Sean Christopherson
2020-03-20 13:55                       ` Nathaniel McCallum
2020-03-15  1:25     ` Jarkko Sakkinen
2020-03-15 17:53       ` Nathaniel McCallum
2020-03-16 13:31         ` Jethro Beekman
2020-03-16 13:57           ` Nathaniel McCallum
2020-03-16 13:59             ` Jethro Beekman
2020-03-16 14:03               ` Nathaniel McCallum
2020-03-16 17:17                 ` Sean Christopherson
2020-03-16 21:27             ` Jarkko Sakkinen
2020-03-16 21:29               ` Jarkko Sakkinen
2020-03-16 22:55           ` Sean Christopherson
2020-03-16 23:56             ` Xing, Cedric
2020-03-18 22:01               ` Jarkko Sakkinen
2020-03-18 22:18                 ` Jarkko Sakkinen
2020-03-16 13:56         ` Jarkko Sakkinen
2020-03-16 14:01           ` Nathaniel McCallum
2020-03-16 21:38             ` Jarkko Sakkinen
2020-03-16 22:53               ` Sean Christopherson
2020-03-16 23:50                 ` Xing, Cedric
2020-03-16 23:59                   ` Sean Christopherson
2020-03-17  0:18                     ` Xing, Cedric
2020-03-17  0:27                       ` Sean Christopherson
2020-03-17 16:37                         ` Nathaniel McCallum
2020-03-17 16:50                       ` Nathaniel McCallum
2020-03-17 21:40                         ` Xing, Cedric
2020-03-17 22:09                           ` Sean Christopherson
2020-03-17 22:36                             ` Xing, Cedric
2020-03-17 23:57                               ` Sean Christopherson
2020-03-17 22:23                         ` Xing, Cedric
2020-03-18 13:01                           ` Nathaniel McCallum
2020-03-20 15:53                             ` Nathaniel McCallum [this message]
2020-03-17 16:28                 ` Nathaniel McCallum
2020-03-18 22:58                   ` Jarkko Sakkinen
2020-03-18 22:39                 ` Jarkko Sakkinen
2020-03-11 19:30   ` Nathaniel McCallum
2020-03-13  0:52     ` Sean Christopherson
2020-03-13 16:07       ` Nathaniel McCallum
2020-03-13 16:33         ` Sean Christopherson
2020-03-03 23:36 ` [PATCH v28 22/22] selftests/x86: Add vDSO selftest for SGX Jarkko Sakkinen
2020-03-04 19:24 ` [PATCH v28 00/22] Intel SGX foundations Nathaniel McCallum
2020-03-17 16:00 ` Jordan Hand
2020-03-18 21:56   ` Jarkko Sakkinen
2020-03-19 17:16   ` Dr. Greg

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