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From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Nathaniel McCallum <npmccallum@redhat.com>
Cc: Jethro Beekman <jethro@fortanix.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>,
	cedric.xing@intel.com, Patrick Uiterwijk <puiterwijk@redhat.com>,
	Andy Lutomirski <luto@amacapital.net>,
	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, 13 Mar 2020 15:08:20 -0700
Message-ID: <20200313220820.GE5181@linux.intel.com> (raw)
In-Reply-To: <CAOASepP_oGOenjCvAvLg+e+=fz4H0X=cyD+v5ywD0peeXEEmYg@mail.gmail.com>

On Fri, Mar 13, 2020 at 04:14:01PM -0400, Nathaniel McCallum wrote:
> On Fri, Mar 13, 2020 at 2:45 PM Sean Christopherson
> <sean.j.christopherson@intel.com> wrote:
 > > This doesn't compromise the ability to treat __vsdo...() like ENCLU if
> > > you need the full power. But it does make it significantly easier to
> > > consume when you don't have special needs. So as I see it, __vdso...()
> > > should:
> > >
> > > 1. preserve %rbx
> > > 2. take leaf in %rcx
> > > 3. gain a void* stack param which is passed to the handler
> >
> > Unless I'm misunderstanding the request, this already exists.  %rsp at the
> > time of EEXIT is passed to the handler.
> 
> Sorry, different stack parameter. I mean this:
> 
> typedef int (*sgx_enclave_exit_handler_t)(
>     long rdi,
>     long rsi,
>     long rdx,
>     long ursp,
>     long r8,
>     long r9,
>     int ret,
>     void *tcs,
>     struct sgx_enclave_exception *e,
>     void *misc
> );
> 
> int __vdso_sgx_enter_enclave(
>     long rdi,
>     long rsi,
>     long rdx,
>     int leaf,
>     long r8,
>     long r9,
>     void *tcs,
>     struct sgx_enclave_exception *e,
>     void *misc,
>     sgx_enclave_exit_handler_t handler
> );
> 
> This is so that the caller of __vdso...() can pass context into the
> handler.

Hrm, I'm not a fan of adding a param that is only consumed by the handler,
especially when there are multiple alternatives, e.g. fudge the param in
assembly before calling __vdso(), have the enclave supply the context in a
volatile register, etc...

> Note that I've also reordered the stack parameters so that the stack
> order can be reused.

Ah, ret<->tcs, took me a minute...

Does preserving tsc->e->misc ordering matter all that much?  __vdso() needs
to manually copy them either way.  I ask because putting @misc at the end
would allow implementations that don't use @handler to omit the param (if
I've done my math correctly, which is always a big if).  That would make
adding the handler-only param a little more palatable.

> > > 4. sub/add to %rsp rather than save/restore
> >
> > Can you elaborate on why you want to sub/add to %rsp instead of having the
> > enclave unwind the stack?  Preserving %rsp across EEXIT/ERESUME seems more
> > in line with function call semantics, which I assume is desirable?  E.g.
> >
> >   push param3
> >   push param2
> >   push param1
> >
> >   enclu[EEXIT]
> >
> >   add $0x18, %rsp
> 
> Before enclave EEXIT, the enclave restores %rsp to the value it had
> before EENTER was called. Then it pushes additional output arguments
> onto the stack. The enclave calls EENCLU[EEXIT].
> 
> We are now in __vdso...() on the way back to the caller. However, %rsp
> has a different value than we entered the function with. This breaks
> x86_64 ABI, obviously. The handler needs to fix this up: how does it
> do so?
> 
> In the current code, __vdso..() saves the value of %rsp, calls the
> handler and then restores %rsp. The handler can fix up the stack by
> setting the correct value to %rbx and returning without restoring it.

Ah, you're referring to the patch where the handler decides to return all
the way back to the caller of __vdso...().

> But this requires internal knowledge of the __vdso...() function,
> which could theoretically change in the future.
> 
> If instead the __vdso...() only did add/sub, then the handler could do:
> 1. pop return address
> 2. pop handler stack params
> 3. pop enclave additional output stack params
> 4. push handler stack params
> 5. push return address
> 
> While this is more work, it is standard calling convention work that
> doesn't require internal knowledge of __vdso..(). Alternatively, if we
> don't like the extra work, we can document the %rbx hack explicitly
> into the handler documentation and make it part of the interface. But
> we need some explicit way for the handler to pop enclave output stack
> params that doesn't depend on internal knowledge of the __vdso...()
> invariants.

IIUC, this is what you're suggesting?  Having to align the stack makes this
a bit annoying, but it's not bad by any means.

diff --git a/arch/x86/entry/vdso/vsgx_enter_enclave.S b/arch/x86/entry/vdso/vsgx_enter_enclave.S
index 94a8e5f99961..05d54f79b557 100644
--- a/arch/x86/entry/vdso/vsgx_enter_enclave.S
+++ b/arch/x86/entry/vdso/vsgx_enter_enclave.S
@@ -139,8 +139,9 @@ SYM_FUNC_START(__vdso_sgx_enter_enclave)
        /* Pass the untrusted RSP (at exit) to the callback via %rcx. */
        mov     %rsp, %rcx

-       /* Save the untrusted RSP in %rbx (non-volatile register). */
+       /* Save the untrusted RSP offset in %rbx (non-volatile register). */
        mov     %rsp, %rbx
+       and     $0xf, %rbx

        /*
         * Align stack per x86_64 ABI. Note, %rsp needs to be 16-byte aligned
@@ -161,8 +162,8 @@ SYM_FUNC_START(__vdso_sgx_enter_enclave)
        mov     0x20(%rbp), %rax
        call    .Lretpoline

-       /* Restore %rsp to its post-exit value. */
-       mov     %rbx, %rsp
+       /* Undo the post-exit %rsp adjustment. */
+       lea     0x20(%rsp,%rbx), %rsp


That's reasonable, let's the handler play more games with minimal overhead.

> > > That would make this a very usable and fast interface without
> > > sacrificing any of its current power.
> >
> 

  reply index

Thread overview: 109+ 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
     [not found]         ` <20200311164047.GG21852@linux.intel.com>
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 [this message]
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
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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