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Date:   Fri, 9 Aug 2019 08:24:04 -0700
From:   Sean Christopherson <sean.j.christopherson@intel.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Cc:     "Ayoun, Serge" <serge.ayoun@intel.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>,
        "linux-sgx@vger.kernel.org" <linux-sgx@vger.kernel.org>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "Hansen, Dave" <dave.hansen@intel.com>,
        "nhorman@redhat.com" <nhorman@redhat.com>,
        "npmccallum@redhat.com" <npmccallum@redhat.com>,
        "Katz-zamir, Shay" <shay.katz-zamir@intel.com>,
        "Huang, Haitao" <haitao.huang@intel.com>,
        "andriy.shevchenko@linux.intel.com" 
        <andriy.shevchenko@linux.intel.com>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "Svahn, Kai" <kai.svahn@intel.com>, "bp@alien8.de" <bp@alien8.de>,
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        "luto@kernel.org" <luto@kernel.org>,
        "Huang, Kai" <kai.huang@intel.com>,
        "rientjes@google.com" <rientjes@google.com>,
        "Xing, Cedric" <cedric.xing@intel.com>
Subject: Re: [PATCH v21 16/28] x86/sgx: Add the Linux SGX Enclave Driver
Message-ID: <20190809152403.GA10541@linux.intel.com>
References: <20190713170804.2340-1-jarkko.sakkinen@linux.intel.com>
 <20190713170804.2340-17-jarkko.sakkinen@linux.intel.com>
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 <20190807151534.kxsletvhbn3lno6w@linux.intel.com>
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 <cb8a7d13a31588205d1c18521e58a0ee4bac03f8.camel@linux.intel.com>
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On Fri, Aug 09, 2019 at 06:02:08PM +0300, Jarkko Sakkinen wrote:
> On Thu, 2019-08-08 at 08:40 -0700, Sean Christopherson wrote:
> > On Wed, Aug 07, 2019 at 06:15:34PM +0300, Jarkko Sakkinen wrote:
> > > On Mon, Jul 29, 2019 at 11:17:57AM +0000, Ayoun, Serge wrote:
> > > > > +	/* TCS pages need to be RW in the PTEs, but can be 0 in the EPCM. */
> > > > > +	if ((secinfo.flags & SGX_SECINFO_PAGE_TYPE_MASK) ==
> > > > > SGX_SECINFO_TCS)
> > > > > +		prot |= PROT_READ | PROT_WRITE;
> > > > 
> > > > For TCS pages you add both RD and WR maximum protection bits.
> > > > For the enclave to be able to run, user mode will have to change the
> > > > "vma->vm_flags" from PROT_NONE to PROT_READ | PROT_WRITE (otherwise
> > > > eenter fails).  This is exactly what your selftest  does.
> > > 
> > > Recap where the TCS requirements came from? Why does it need
> > > RW in PTEs and can be 0 in the EPCM? The comment should explain
> > > it rather leave it as a claim IMHO.
> > 
> > Hardware ignores SECINFO.FLAGS.{R,W,X} coming from userspace and instead
> > forces RWX=0.  It does this to prevent software from directly accessing
> > the TCS.  But hardware still accesses the TCS through a virtual address,
> > e.g. to allow software to zap the page for reclaim, which means hardware
> > generates reads and writes to the TCS, i.e. the PTEs need RW permissions.
> 
> Manipulating a PTE should not require any specific permissions on the
> page that it is defining. Why RW is required in SGX context?

By PTEs I meant the TCS needs to be mapped RW in the kernel's page tables,
e.g. hardware generates read and write accesses to the TCS when entering
an enclave.

> > So, for the EADD ioctl(), it's not unreasonable for userspace to provide
> > SECINFO.FLAGS.{R,W,X} = 0 for the TCS to match what will actually get
> > jammed into the EPCM.  Allowing userspace to specify RWX=0 means the
> > kernel needs to manually add PROT_READ and PROT_WRITE to the allowed prot
> > bits so that mmap()/mprotect() work as expected.
> 
> Anyway, appreciate your throughtout explanation, thanks.
> 
> /Jarkko
>