linux-kernel - RE: [intel-sgx-kernel-dev] [PATCH v7 4/8] intel_sgx: driver for Intel Software Guard Extensions

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Date:   Fri, 8 Dec 2017 15:31:38 +0000
From:   "Christopherson, Sean J" <sean.j.christopherson@...el.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
CC:     "intel-sgx-kernel-dev@...ts.01.org" 
        <intel-sgx-kernel-dev@...ts.01.org>,
        "platform-driver-x86@...r.kernel.org" 
        <platform-driver-x86@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Darren Hart <dvhart@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Shevchenko <andy@...radead.org>
Subject: RE: [intel-sgx-kernel-dev] [PATCH v7 4/8] intel_sgx: driver for
 Intel Software Guard Extensions

Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com> wrote:
> On Thu, Dec 07, 2017 at 02:46:39PM +0000, Christopherson, Sean J wrote:
> > > +	for (i = 0; i < 2; i++) {
> > > +		va_page = list_first_entry(&encl->va_pages,
> > > +					   struct sgx_va_page, list);
> > > +		va_offset = sgx_alloc_va_slot(va_page);
> > > +		if (va_offset < PAGE_SIZE)
> > > +			break;
> > > +
> > > +		list_move_tail(&va_page->list, &encl->va_pages);
> > > +	}
> > 
> > This is broken, there is no guarantee that the next VA page will have
> > a free slot.  You have to walk over all VA pages to guarantee a slot
> > is found, e.g. this caused EWB and ELDU errors.
> 
> I did run some extensive stress tests on this and did not experience any
> issues. Full VA pages are always put to the end. Please point me to the
> test where this breaks so that I can fix the issue if it persists.

Three VA pages in the enclave: A, B and C.  Evict all pages in the
enclave, i.e. consume all slots in A, B and C.  The list can be in
any order at this point, but for the sake of argument let's say the
order is C->A->B, i.e. C was originally the last VA page in the list.
Fault in page X, whose VA is in B.  Evict X.  This code looks at C
and A, and finds no available slot, but continues with VA page A and
a va_offset of PAGE_SIZE.