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

Haitao Huang
2022-03-17T14:28:52Z

Hi

On Thu, 17 Mar 2022 02:11:28 -0500, Jarkko Sakkinen <jarkko@kernel.org>
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@kernel.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.
>> >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@intel.com/

Thanks
Haitao