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

[Haitao Huang]

Hi

On Thu, 17 Mar 2022 02:11:28 -0500, Jarkko Sakkinen <jarkko@xxxxxxxxxx>  
wrote:

> 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@xxxxxxxxxx>
> > > 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.
> > >ar
> > > 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.

Jarkko, how would automatically forcing PROT_READ on #PF work for this  
sequence?

1) EAUG a page (has to be RW)
2) EACCEPT(RW)
3) enclave copies some data to page
4) enclave wants to change permission to R

If you are proposing mprotect, then as I indicated earlier, please address  
concerns raised by Reinette:
https://lore.kernel.org/linux-sgx/e1c04077-0165-c5ec-53be-7fd732965e80@xxxxxxxxx/



Thanks
Haitao