linux-kernel - Re: [RFC PATCH v2 2/5] x86/sgx: Require userspace to define enclave pages' protection bits

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Message-ID: <20190610181744.GH15995@linux.intel.com>
Date:   Mon, 10 Jun 2019 11:17:44 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Cedric Xing <cedric.xing@...el.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        James Morris <jmorris@...ei.org>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        Paul Moore <paul@...l-moore.com>,
        Eric Paris <eparis@...isplace.org>, selinux@...r.kernel.org,
        Jethro Beekman <jethro@...tanix.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        linux-sgx@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>, nhorman@...hat.com,
        npmccallum@...hat.com, Serge Ayoun <serge.ayoun@...el.com>,
        Shay Katz-zamir <shay.katz-zamir@...el.com>,
        Haitao Huang <haitao.huang@...el.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Kai Svahn <kai.svahn@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        Josh Triplett <josh@...htriplett.org>,
        Kai Huang <kai.huang@...el.com>,
        David Rientjes <rientjes@...gle.com>,
        William Roberts <william.c.roberts@...el.com>,
        Philip Tricca <philip.b.tricca@...el.com>
Subject: Re: [RFC PATCH v2 2/5] x86/sgx: Require userspace to define enclave
 pages' protection bits

On Mon, Jun 10, 2019 at 08:45:06PM +0300, Jarkko Sakkinen wrote:
> On Mon, Jun 10, 2019 at 09:15:33AM -0700, Sean Christopherson wrote:
> > > 'flags' should would renamed as 'secinfo_flags_mask' even if the name is
> > > longish. It would use the same values as the SECINFO flags. The field in
> > > struct sgx_encl_page should have the same name. That would express
> > > exactly relation between SECINFO and the new field. I would have never
> > > asked on last iteration why SECINFO is not enough with a better naming.
> > 
> > No, these flags do not impact the EPCM protections in any way.  Userspace
> > can extend the EPCM protections without going through the kernel.  The
> > protection flags for an enclave page impact VMA/PTE protection bits.
> > 
> > IMO, it is best to treat the EPCM as being completely separate from the
> > kernel's EPC management.
> 
> It is a clumsy API if permissions are not taken in the same format for
> everything. There is no reason not to do it. The way mprotect() callback
> just interprets the field is as VMA permissions.

They are two entirely different things.  The explicit protection bits are
consumed by the kernel, while SECINFO.flags is consumed by the CPU.  The
intent is to have the protection flags be analogous to mprotect(), the
fact that they have a similar/identical format to SECINFO is irrelevant.

Calling the field secinfo_flags_mask is straight up wrong on SGX2, as 
userspace can use EMODPE to set SECINFO after the page is added.  It's
also wrong on SGX1 when adding TCS pages since SECINFO.RWX bits for TCS
pages are forced to zero by hardware.

> It would also be more future-proof just to have a mask covering all bits
> of the SECINFO flags field.

This simply doesn't work, e.g. the PENDING, MODIFIED and PR flags in the
SECINFO are read-only from a software perspective.