linux-kernel - Re: [PATCH V2 16/32] x86/sgx: Support restricting of enclave page permissions

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Date:   Thu, 17 Mar 2022 09:11:28 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Haitao Huang <haitao.huang@...ux.intel.com>
Cc:     Reinette Chatre <reinette.chatre@...el.com>,
        "Dhanraj, Vijay" <vijay.dhanraj@...el.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "bp@...en8.de" <bp@...en8.de>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "Huang, Kai" <kai.huang@...el.com>,
        "Zhang, Cathy" <cathy.zhang@...el.com>,
        "Xing, Cedric" <cedric.xing@...el.com>,
        "Huang, Haitao" <haitao.huang@...el.com>,
        "Shanahan, Mark" <mark.shanahan@...el.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        nathaniel@...fian.com
Subject: Re: [PATCH V2 16/32] x86/sgx: Support restricting of enclave page
 permissions

On Thu, Mar 17, 2022 at 09:01:07AM +0200, Jarkko Sakkinen wrote:
> On Mon, Mar 14, 2022 at 10:39:36AM -0500, Haitao Huang wrote:
> > Hi Jarkko
> > 
> > On Sun, 13 Mar 2022 21:58:51 -0500, Jarkko Sakkinen <jarkko@...nel.org>
> > wrote:
> > 
> > > On Mon, Mar 14, 2022 at 04:50:56AM +0200, Jarkko Sakkinen wrote:
> > > > On Mon, Mar 14, 2022 at 04:49:37AM +0200, Jarkko Sakkinen wrote:
> > > > > On Fri, Mar 11, 2022 at 09:53:29AM -0800, Reinette Chatre wrote:
> > > > >
> > > > > > I saw Haitao's note that EMODPE requires "Read access permitted
> > > > by enclave".
> > > > > > This motivates that EMODPR->PROT_NONE should not be allowed
> > > > since it would
> > > > > > not be possible to relax permissions (run EMODPE) after that.
> > > > Even so, I
> > > > > > also found in the SDM that EACCEPT has the note "Read access
> > > > permitted
> > > > > > by enclave". That seems to indicate that EMODPR->PROT_NONE is
> > > > not practical
> > > > > > from that perspective either since the enclave will not be able to
> > > > > > EACCEPT the change. Does that match your understanding?
> > > > >
> > > > > Yes, PROT_NONE should not be allowed.
> > > > >
> > > > > This is however the real problem.
> > > > >
> > > > > The current kernel patch set has inconsistent API and EMODPR ioctl is
> > > > > simply unacceptable. It  also requires more concurrency management
> > > > from
> > > > > user space run-time, which would be heck a lot easier to do in the
> > > > kernel.
> > > > >
> > > > > If you really want EMODPR as ioctl, then for consistencys sake,
> > > > then EAUG
> > > > > should be too. Like this when things go opposite directions, this
> > > > patch set
> > > > > plain and simply will not work out.
> > > > >
> > > > > I would pick EAUG's strategy from these two as it requires half
> > > > the back
> > > > > calls to host from an enclave. I.e. please combine mprotect() and
> > > > EMODPR,
> > > > > either in the #PF handler or as part of mprotect(), which ever
> > > > suits you
> > > > > best.
> > > > >
> > > > > I'll try demonstrate this with two examples.
> > > > >
> > > > > mmap() could go something like this() (simplified):
> > > > > 1. Execution #UD's to SYSCALL.
> > > > > 2. Host calls enclave's mmap() handler with mmap() parameters.
> > > > > 3. Enclave up-calls host's mmap().
> > > > > 4. Loops the range with EACCEPTCOPY.
> > > > >
> > > > > mprotect() has to be done like this:
> > > > > 1. Execution #UD's to SYSCALL.
> > > > > 2. Host calls enclave's mprotect() handler.
> > > > > 3. Enclave up-calls host's mprotect().
> > > > > 4. Enclave up-calls host's ioctl() to SGX_IOC_ENCLAVE_PERMISSIONS.
> > 
> > I assume up-calls here are ocalls as we call them in our implementation,
> > which are the calls enclave make to untrusted side via EEXIT.
> > 
> > If so, can your implementation combine this two up-calls into one, then host
> > side just do ioctl() and mprotect to kernel? If so, would that address your
> > concern about extra up-calls?
> > 
> > 
> > > > > 3. Loops the range with EACCEPT.
> > > >   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > >   5. Loops the range with EACCEPT + EMODPE.
> > > > 
> > > > > This is just terrible IMHO. I hope these examples bring some insight.
> > > 
> > > E.g. in Enarx we have to add a special up-call (so called enarxcall in
> > > intermediate that we call sallyport, which provides shared buffer to
> > > communicate with the enclave) just for reseting the range with PROT_READ.
> > > Feel very redundant, adds ugly cruft and is completely opposite strategy
> > > to
> > > what you've chosen to do with EAUG, which is I think correct choice as
> > > far
> > > as API is concerned.
> > 
> > The problem with EMODPR on #PF is that kernel needs to know what permissions
> > requested from enclave at the time of #PF. So enclave has to make at least
> > one call to kernel (again via ocall in our case, I assume up-call in your
> > case) to make the change.
> 
> The #PF handler should do unconditionally EMODPR with PROT_READ.

Or mprotect(), as long as secinfo contains PROT_READ. I don't care about
this detail hugely anymore because it does not affect uapi. 

Using EMODPR as a permission control mechanism is a ridiculous idea, and
I cannot commit to maintain a broken uapi.

BR, Jarkko