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

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Date:   Wed, 12 Jun 2019 22:26:26 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Sean Christopherson <sean.j.christopherson@...el.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 11:17:44AM -0700, Sean Christopherson wrote:
> 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.

The new variable tells the limits on which kernel will co-operate with
the enclave. It is way more descriptive than 'flags'.

> > 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.

It is easy to validate reserved bits from a SECINFO struct.

/Jarkko