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Message-ID: <209cf4a5-eda9-2495-539f-fed22252cf02@intel.com>
Date: Tue, 6 Nov 2018 13:00:49 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Andy Lutomirski <luto@...capital.net>
Cc: Andy Lutomirski <luto@...nel.org>,
"Christopherson, Sean J" <sean.j.christopherson@...el.com>,
Jann Horn <jannh@...gle.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Rich Felker <dalias@...c.org>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Jethro Beekman <jethro@...tanix.com>,
Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
Florian Weimer <fweimer@...hat.com>,
Linux API <linux-api@...r.kernel.org>, X86 ML <x86@...nel.org>,
linux-arch <linux-arch@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>,
Peter Zijlstra <peterz@...radead.org>, nhorman@...hat.com,
npmccallum@...hat.com, "Ayoun, Serge" <serge.ayoun@...el.com>,
shay.katz-zamir@...el.com, linux-sgx@...r.kernel.org,
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
Carlos O'Donell <carlos@...hat.com>,
adhemerval.zanella@...aro.org
Subject: Re: RFC: userspace exception fixups
On 11/6/18 12:12 PM, Andy Lutomirski wrote:
> True, but what if we have a nasty enclave that writes to memory just
> below SP *before* decrementing SP?
Yeah, that would be unfortunate. If an enclave did this (roughly):
1. EENTER
2. Hardware sets eenter_hwframe->sp = %sp
3. Enclave runs... wants to do out-call
4. Enclave sets up parameters:
memcpy(&eenter_hwframe->sp[-offset], arg1, size);
...
5. Enclave sets eenter_hwframe->sp -= offset
If we got a signal between 4 and 5, we'd clobber the copy of 'arg1' that
was on the stack. The enclave could easily fix this by moving ->sp first.
But, this is one of those "fun" parts of the ABI that I think we need to
talk about. If we do this, we also basically require that the code
which handles asynchronous exits must *not* write to the stack. That's
not hard because it's typically just a single ERESUME instruction, but
it *is* a requirement.
It means fun stuff like that you absolutely can't just async-exit to C code.
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