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From: Nathaniel McCallum <npmccallum@redhat.com>
To: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: 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>,
	Jethro Beekman <jethro@fortanix.com>
Subject: Re: [PATCH v28 21/22] x86/vdso: Implement a vDSO for Intel SGX enclave call
Date: Fri, 13 Mar 2020 12:07:55 -0400
Message-ID: <CAOASepMN1fmDaPjJJ-rpbLNPGUzE2LNB69s3X5mDBEhXziZ_UQ@mail.gmail.com> (raw)
In-Reply-To: <20200313005252.GA1292@linux.intel.com>

On Thu, Mar 12, 2020 at 8:52 PM Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> On Wed, Mar 11, 2020 at 03:30:44PM -0400, Nathaniel McCallum wrote:
> > On Tue, Mar 3, 2020 at 6:40 PM Jarkko Sakkinen
> > <jarkko.sakkinen@linux.intel.com> wrote:
> > > + * The exit handler's return value is interpreted as follows:
> > > + *  >0:                continue, restart __vdso_sgx_enter_enclave() with @ret as @leaf
> > > + *   0:                success, return @ret to the caller
> > > + *  <0:                error, return @ret to the caller
> > > + *
> > > + * The userspace exit handler is responsible for unwinding the stack, e.g. to
> > > + * pop @e, u_rsp and @tcs, prior to returning to __vdso_sgx_enter_enclave().
> >
> > Unless I misunderstand, this documentation...
>
> Hrm, that does appear wrong.  I'm guessing that was leftover from a previous
> incarnation of the code.  Or I botched the description, which is just as
> likely.

I figured out what happened on my end. This documentation error led me
to misread the code. More below.

> > > + * The exit handler may also transfer control, e.g. via longjmp() or a C++
> > > + * exception, without returning to __vdso_sgx_enter_enclave().
> > > + *
> > > + * Return:
> > > + *  0 on success,
> > > + *  -EINVAL if ENCLU leaf is not allowed,
> > > + *  -EFAULT if an exception occurs on ENCLU or within the enclave
> > > + *  -errno for all other negative values returned by the userspace exit handler
> > > + */
>
> ...
>
> > > +       /* Load the callback pointer to %rax and invoke it via retpoline. */
> > > +       mov     0x20(%rbp), %rax
> > > +       call    .Lretpoline
> > > +
> > > +       /* Restore %rsp to its post-exit value. */
> > > +       mov     %rbx, %rsp
> >
> > ... doesn't seem to match this code.
> >
> > If the handler pops from the stack and then we restore the stack here,
> > the handler had no effect.
> >
> > Also, one difference between this interface and a raw ENCLU[EENTER] is
> > that we can't pass arguments on the untrusted stack during EEXIT. If
> > we want to support that workflow, then we need to allow the ability
> > for the handler to pop "additional" values without restoring the stack
> > pointer to the exact value here.
>
> > Also, one difference between this interface and a raw ENCLU[EENTER] is
> > that we can't pass arguments on the untrusted stack during EEXIT. If
> > we want to support that workflow, then we need to allow the ability
> > for the handler to pop "additional" values without restoring the stack
> > pointer to the exact value here.
>
> The callback shenanigans exist precisely to allow passing arguments on the
> untrusted stack.  The vDSO is very careful to preserve the stack memory
> above RSP, and to snapshot RSP at the time of exit, e.g. the arguments in
> memory and their addresses relative to u_rsp live across EEXIT.  It's the
> same basic concept as regular function calls, e.g. the callee doesn't pop
> params off the stack, it just knows what addresses relative to RSP hold
> the data it wants.  The enclave, being the caller, is responsible for
> cleaning up u_rsp.
>
> FWIW, if the handler reaaaly wanted to pop off the stack, it could do so,
> fixup the stack, and then re-call __vdso_sgx_enter_enclave() instead of
> returning (to the original __vdso_sgx_enter_enclave()).

My understanding from the documentation issue above was that *if* you
wanted to push parameters back on the stack during enclave exit, you
would *have* to supply a handler so it could pop the parameters and
reset the stack. Which is why restoring %rsp from %rbx didn't make
sense to me.

Related to my other message in this thread, if
__vdso_sgx_enter_enclave() preserved %rbx and took @leaf as a stack
parameter, you could call __vdso_sgx_enter_enclave() from C so long as
the enclave didn't push return arguments on the stack. A workaround
for that case would be to fix up the stack in the handler. It would be
enough for the handler to simply set %rbx to the desired stack
location and return (though all of this unclean of course).

> > > +       /*
> > > +        * If the return from callback is zero or negative, return immediately,
> > > +        * else re-execute ENCLU with the postive return value interpreted as
> > > +        * the requested ENCLU leaf.
> > > +        */
> > > +       cmp     $0, %eax
> > > +       jle     .Lout
> > > +       jmp     .Lenter_enclave
> > > +
> > > +.Lretpoline:
> > > +       call    2f
> > > +1:     pause
> > > +       lfence
> > > +       jmp     1b
> > > +2:     mov     %rax, (%rsp)
> > > +       ret
> > > +       .cfi_endproc
> > > +
> > > +_ASM_VDSO_EXTABLE_HANDLE(.Lenclu_eenter_eresume, .Lhandle_exception)
>


  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
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 [this message]
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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