From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: Dave Hansen <dave.hansen@intel.com>, Jann Horn <jannh@google.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Rich Felker <dalias@libc.org>,
Dave Hansen <dave.hansen@linux.intel.com>,
Jethro Beekman <jethro@fortanix.com>,
Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
Florian Weimer <fweimer@redhat.com>,
Linux API <linux-api@vger.kernel.org>, X86 ML <x86@kernel.org>,
linux-arch <linux-arch@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
nhorman@redhat.com, npmccallum@redhat.com, "Ayoun,
Serge" <serge.ayoun@intel.com>,
shay.katz-zamir@intel.com, linux-sgx@vger.kernel.org,
Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Carlos O'Donell <carlos@redhat.com>,
adhemerval.zanella@linaro.org
Subject: Re: RFC: userspace exception fixups
Date: Wed, 7 Nov 2018 07:34:52 -0800 [thread overview]
Message-ID: <20181107153452.GB22972@linux.intel.com> (raw)
Message-ID: <20181107153452.Ioda4IWBl93jVX9ZHCAtrgp4U7VUlQl09X9jjFsrzL0@z> (raw)
In-Reply-To: <CALCETrV_sMOeQGRkxoA8KMxEizagB9bD-mB2T-ro3F6O6OX9Xw@mail.gmail.com>
On Tue, Nov 06, 2018 at 05:17:14PM -0800, Andy Lutomirski wrote:
> On Tue, Nov 6, 2018 at 4:02 PM Sean Christopherson
> <sean.j.christopherson@intel.com> wrote:
> >
> > On Tue, Nov 06, 2018 at 03:39:48PM -0800, Andy Lutomirski wrote:
> > > On Tue, Nov 6, 2018 at 3:35 PM Sean Christopherson
> > > <sean.j.christopherson@intel.com> wrote:
> > > >
> > > > On Tue, Nov 06, 2018 at 03:00:56PM -0800, Andy Lutomirski wrote:
> > > > >
> > > > >
> > > > > >> On Nov 6, 2018, at 1:59 PM, Sean Christopherson <sean.j.christopherson@intel.com> wrote:
> > > > > >>
> > > > > >>> On Tue, 2018-11-06 at 13:41 -0800, Andy Lutomirski wrote:
> > > > > >> Sean, how does the current SDK AEX handler decide whether to do
> > > > > >> EENTER, ERESUME, or just bail and consider the enclave dead? It seems
> > > > > >> like the *CPU* could give a big hint, but I don't see where there is
> > > > > >> any architectural indication of why the AEX code got called or any
> > > > > >> obvious way for the user code to know whether the exit was fixed up by
> > > > > >> the kernel?
> > > > > >
> > > > > > The SDK "unconditionally" does ERESUME at the AEP location, but that's
> > > > > > bit misleading because its signal handler may muck with the context's
> > > > > > RIP, e.g. to abort the enclave on a fatal fault.
> > > > > >
> > > > > > On an event/exception from within an enclave, the event is immediately
> > > > > > delivered after loading synthetic state and changing RIP to the AEP.
> > > > > > In other words, jamming CPU state is essentially a bunch of vectoring
> > > > > > ucode preamble, but from software's perspective it's a normal event
> > > > > > that happens to point at the AEP instead of somewhere in the enclave.
> > > > > > And because the signals the SDK cares about are all synchronous, the
> > > > > > SDK can simply hardcode ERESUME at the AEP since all of the fault logic
> > > > > > resides in its signal handler. IRQs and whatnot simply trampoline back
> > > > > > into the enclave.
> > > > > >
> > > > > > Userspace can do something funky instead of ERESUME, but only *after*
> > > > > > IRET/RSM/VMRESUME has returned to the AEP location, and in Linux's
> > > > > > case, after the trap handler has run.
> > > > > >
> > > > > > Jumping back a bit, how much do we care about preventing userspace
> > > > > > from doing stupid things?
> > > > >
> > > > > My general feeling is that userspace should be allowed to do apparently
> > > > > stupid things. For example, as far as the kernel is concerned, Wine and
> > > > > DOSEMU are just user programs that do stupid things. Linux generally tries
> > > > > to provide a reasonably complete view of architectural behavior. This is
> > > > > in contrast to, say, Windows, where IIUC doing an unapproved WRFSBASE May
> > > > > cause very odd behavior indeed. So magic fixups that do non-architectural
> > > > > things are not so great.
> > > >
> > > > Sorry if I'm beating a dead horse, but what if we only did fixup on ENCLU
> > > > with a specific (ignored) prefix pattern? I.e. effectively make the magic
> > > > fixup opt-in, falling back to signals. Jamming RIP to skip ENCLU isn't
> > > > that far off the architecture, e.g. EENTER stuffs RCX with the next RIP so
> > > > that the enclave can EEXIT to immediately after the EENTER location.
> > > >
> > >
> > > How does that even work, though? On an AEX, RIP points to the ERESUME
> > > instruction, not the EENTER instruction, so if we skip it we just end
> > > up in lala land.
> >
> > Userspace would obviously need to be aware of the fixup behavior, but
> > it actually works out fairly nicely to have a separate path for ERESUME
> > fixup since a fault on EENTER is generally fatal, whereas as a fault on
> > ERESUME might be recoverable.
> >
>
> Hmm.
>
> >
> > do_eenter:
> > mov tcs, %rbx
> > lea async_exit, %rcx
> > mov $EENTER, %rax
> > ENCLU
>
> Or SOME_SILLY_PREFIX ENCLU?
Yeah, forgot to include that.
> >
> > /*
> > * EEXIT or EENTER faulted. In the latter case, %RAX already holds some
> > * fault indicator, e.g. -EFAULT.
> > */
> > eexit_or_eenter_fault:
> > ret
>
> But userspace wants to know whether it was a fault or not. So I think
> we either need two landing pads or we need to hijack a flag bit (are
> there any known-zeroed flag bits after EEXIT?) to say whether it was a
> fault. And, if it was a fault, we should give the vector, the
> sanitized error code, and possibly CR2.
As Jethro mentioned, RAX will always be 4 on a successful EEXIT, so we
can use RAX to indicate a fault. That's what I was trying to imply with
EFAULT. Here's the reg stuffing I use for the POC:
regs->ax = EFAULT;
regs->di = trapnr;
regs->si = error_code;
regs->dx = address;
Well-known RAX values also means the kernel fault handlers only need to
look for SOME_SILLY_PREFIX ENCLU if RAX==2 || RAX==3, i.e. the fault
occurred on EENTER or in an enclave (RAX is set to ERESUME's leaf as
part of the asynchronous enlcave exit flow).
> >
> > async_exit:
> > ENCLU
>
> Same prefix here, right?
>
> >
> > fixup_handler:
> > <do fault stuff>
>
> This whole thing is a bit odd, but not necessarily a terrible idea.
next prev parent reply other threads:[~2018-11-07 15:34 UTC|newest]
Thread overview: 163+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-01 17:53 RFC: userspace exception fixups Andy Lutomirski
2018-11-01 17:53 ` Andy Lutomirski
2018-11-01 18:09 ` Florian Weimer
2018-11-01 18:09 ` Florian Weimer
2018-11-01 18:30 ` Rich Felker
2018-11-01 18:30 ` Rich Felker
2018-11-01 19:00 ` Jarkko Sakkinen
2018-11-01 19:00 ` Jarkko Sakkinen
2018-11-01 18:27 ` Rich Felker
2018-11-01 18:27 ` Rich Felker
2018-11-01 18:33 ` Jann Horn
2018-11-01 18:33 ` Jann Horn
2018-11-01 18:52 ` Rich Felker
2018-11-01 18:52 ` Rich Felker
2018-11-01 19:10 ` Linus Torvalds
2018-11-01 19:10 ` Linus Torvalds
2018-11-01 19:31 ` Rich Felker
2018-11-01 19:31 ` Rich Felker
2018-11-01 21:24 ` Linus Torvalds
2018-11-01 21:24 ` Linus Torvalds
2018-11-01 23:22 ` Andy Lutomirski
2018-11-01 23:22 ` Andy Lutomirski
2018-11-02 16:30 ` Sean Christopherson
2018-11-02 16:30 ` Sean Christopherson
2018-11-02 16:37 ` Jethro Beekman
2018-11-02 16:37 ` Jethro Beekman
2018-11-02 16:52 ` Sean Christopherson
2018-11-02 16:52 ` Sean Christopherson
2018-11-02 16:56 ` Jethro Beekman
2018-11-02 16:56 ` Jethro Beekman
2018-11-02 17:01 ` Andy Lutomirski
2018-11-02 17:01 ` Andy Lutomirski
2018-11-02 17:05 ` Jethro Beekman
2018-11-02 17:05 ` Jethro Beekman
2018-11-02 17:16 ` Andy Lutomirski
2018-11-02 17:16 ` Andy Lutomirski
2018-11-02 17:32 ` Rich Felker
2018-11-02 17:32 ` Rich Felker
2018-11-02 17:12 ` Sean Christopherson
2018-11-02 17:12 ` Sean Christopherson
2018-11-02 22:42 ` Jarkko Sakkinen
2018-11-02 22:42 ` Jarkko Sakkinen
2018-11-02 16:56 ` Dave Hansen
2018-11-02 16:56 ` Dave Hansen
2018-11-02 17:06 ` Sean Christopherson
2018-11-02 17:06 ` Sean Christopherson
2018-11-02 17:13 ` Dave Hansen
2018-11-02 17:13 ` Dave Hansen
2018-11-02 17:33 ` Sean Christopherson
2018-11-02 17:33 ` Sean Christopherson
2018-11-02 17:48 ` Andy Lutomirski
2018-11-02 17:48 ` Andy Lutomirski
2018-11-02 18:27 ` Sean Christopherson
2018-11-02 18:27 ` Sean Christopherson
2018-11-02 19:02 ` Jann Horn
2018-11-02 19:02 ` Jann Horn
2018-11-02 22:04 ` Sean Christopherson
2018-11-02 22:04 ` Sean Christopherson
2018-11-02 23:27 ` Jann Horn
2018-11-02 23:27 ` Jann Horn
2018-11-02 23:32 ` Andy Lutomirski
2018-11-02 23:32 ` Andy Lutomirski
2018-11-02 23:36 ` Jann Horn
2018-11-02 23:36 ` Jann Horn
2018-11-06 15:37 ` Sean Christopherson
2018-11-06 15:37 ` Sean Christopherson
2018-11-06 16:57 ` Andy Lutomirski
2018-11-06 16:57 ` Andy Lutomirski
2018-11-06 17:03 ` Dave Hansen
2018-11-06 17:03 ` Dave Hansen
2018-11-06 17:19 ` Sean Christopherson
2018-11-06 17:19 ` Sean Christopherson
2018-11-06 18:20 ` Andy Lutomirski
2018-11-06 18:20 ` Andy Lutomirski
2018-11-06 18:41 ` Dave Hansen
2018-11-06 18:41 ` Dave Hansen
2018-11-06 19:02 ` Andy Lutomirski
2018-11-06 19:02 ` Andy Lutomirski
2018-11-06 19:22 ` Dave Hansen
2018-11-06 19:22 ` Dave Hansen
2018-11-06 20:12 ` Andy Lutomirski
2018-11-06 20:12 ` Andy Lutomirski
2018-11-06 21:00 ` Dave Hansen
2018-11-06 21:00 ` Dave Hansen
2018-11-06 21:07 ` Andy Lutomirski
2018-11-06 21:07 ` Andy Lutomirski
2018-11-06 21:41 ` Andy Lutomirski
2018-11-06 21:41 ` Andy Lutomirski
2018-11-06 21:59 ` Sean Christopherson
2018-11-06 21:59 ` Sean Christopherson
2018-11-06 23:00 ` Andy Lutomirski
2018-11-06 23:00 ` Andy Lutomirski
2018-11-06 23:35 ` Sean Christopherson
2018-11-06 23:35 ` Sean Christopherson
2018-11-06 23:39 ` Andy Lutomirski
2018-11-06 23:39 ` Andy Lutomirski
2018-11-07 0:02 ` Sean Christopherson
2018-11-07 0:02 ` Sean Christopherson
2018-11-07 1:17 ` Andy Lutomirski
2018-11-07 1:17 ` Andy Lutomirski
2018-11-07 6:47 ` Jethro Beekman
2018-11-07 6:47 ` Jethro Beekman
2018-11-07 15:34 ` Sean Christopherson [this message]
2018-11-07 15:34 ` Sean Christopherson
2018-11-07 19:01 ` Sean Christopherson
2018-11-07 19:01 ` Sean Christopherson
2018-11-07 20:56 ` Dave Hansen
2018-11-07 20:56 ` Dave Hansen
2018-11-08 15:04 ` Jarkko Sakkinen
2018-11-08 15:04 ` Jarkko Sakkinen
2018-11-08 19:54 ` Sean Christopherson
2018-11-08 19:54 ` Sean Christopherson
2018-11-08 20:05 ` Andy Lutomirski
2018-11-08 20:05 ` Andy Lutomirski
2018-11-08 20:10 ` Dave Hansen
2018-11-08 20:10 ` Dave Hansen
2018-11-08 21:16 ` Sean Christopherson
2018-11-08 21:16 ` Sean Christopherson
2018-11-08 21:50 ` Dave Hansen
2018-11-08 21:50 ` Dave Hansen
2018-11-08 22:04 ` Sean Christopherson
2018-11-08 22:04 ` Sean Christopherson
2018-11-09 7:12 ` Christoph Hellwig
2018-11-09 7:12 ` Christoph Hellwig
2018-11-06 23:17 ` Rich Felker
2018-11-06 23:17 ` Rich Felker
2018-11-06 23:26 ` Sean Christopherson
2018-11-06 23:26 ` Sean Christopherson
2018-11-07 21:27 ` Rich Felker
2018-11-07 21:27 ` Rich Felker
2018-11-07 21:33 ` Andy Lutomirski
2018-11-07 21:33 ` Andy Lutomirski
2018-11-07 21:40 ` Sean Christopherson
2018-11-07 21:40 ` Sean Christopherson
2018-11-08 15:11 ` Jarkko Sakkinen
2018-11-08 15:11 ` Jarkko Sakkinen
2018-11-06 17:00 ` Dave Hansen
2018-11-06 17:00 ` Dave Hansen
2018-11-02 22:37 ` Jarkko Sakkinen
2018-11-02 22:37 ` Jarkko Sakkinen
2018-11-01 19:06 ` Linus Torvalds
2018-11-01 19:06 ` Linus Torvalds
2018-11-02 22:07 ` Jarkko Sakkinen
2018-11-02 22:07 ` Jarkko Sakkinen
2018-11-18 7:15 ` Jarkko Sakkinen
2018-11-18 7:18 ` Jarkko Sakkinen
2018-11-18 13:02 ` Jarkko Sakkinen
2018-11-19 5:17 ` Jethro Beekman
2018-11-19 14:05 ` Jarkko Sakkinen
2018-11-19 14:59 ` Jarkko Sakkinen
2018-11-19 15:29 ` Andy Lutomirski
2018-11-19 16:02 ` Jarkko Sakkinen
2018-11-19 17:00 ` Andy Lutomirski
2018-11-20 10:11 ` Jarkko Sakkinen
2018-11-20 15:19 ` Andy Lutomirski
2018-11-20 22:55 ` Jarkko Sakkinen
2018-11-21 5:17 ` Jethro Beekman
2018-11-21 15:17 ` Jarkko Sakkinen
2018-11-24 17:07 ` Jarkko Sakkinen
2018-11-26 14:35 ` Sean Christopherson
2018-11-26 22:06 ` Jarkko Sakkinen
2018-11-20 18:09 ` Sean Christopherson
2018-11-20 22:46 ` Jarkko Sakkinen
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