Re: [PATCH v38 10/24] mm: Add vm_ops->mprotect()

[Sean Christopherson]

On Fri, Sep 25, 2020 at 10:18:28AM -0700, Dave Hansen wrote:
> Thanks for the walkthrough.  The thing that clicked for me seeing those
> examples was how the earlier ioctl(ADD_PAGE) is "bound" to later
> enforcement actions at enclave PTE creation time.
> 
> On 9/24/20 5:00 PM, Sean Christopherson wrote:
> > My concern is that if we merge this
> > 
> >         ioctl(sgx_fd, ENCLAVE_ADD_PAGE, SGX_PROT_READ | SGX_PROT_EXEC, ptr, size);
> > 
> > without ->mprotect(), we can't actually enforce the declared protections.  And
> > if we drop the field altogether:
> > 
> >         ioctl(sgx_fd, ENCLAVE_ADD_PAGE, ptr, size);
> > 
> > then we can't implement security_enclave_load().
> 
> To me, it's perfectly OK to have parts of the ABI which are unused.  It
> sure makes them harder to test if there are no actual users in the code,
> but if it solves a real problem with the ABI, I'm fine with it.
> 
> Let's see if I can put all the pieces together.
> 
> Background:
> 
> 1. SGX enclave pages are populated with data by copying data to them
>    from normal memory via: ioctl(sgx_fd, ENCLAVE_ADD_PAGE, src_ptr...);
> 2. We want to be able to restrict those normal memory data sources.  For
>    instance, before copying data to an executable enclave page, we might
>    ensure that the source is executable.
> 3. Enclave page permissions are dynamic just like normal permissions and
>    can be adjusted at runtime with mprotect() (along with a
>    corresponding special instruction inside the enclave)
> 4. The original data source may have have long since vanished at the
>    time when enclave page permission are established (mmap() or
>    mprotect())
> 
> Solution:
> 
> The solution is to force enclaves creators to declare their intent up
> front to ioctl(ENCLAVE_ADD_PAGE).  This intent can me immediately
> compared to the source data mapping (and rejected if necessary).  It is
> also stashed off and then later compared with enclave PTEs to ensure
> that any future mmap()/mprotect() operations performed by the enclave
> creator or the enclave itself are consistent with the earlier declared
> permissions.
> 
> Essentially, this means that whenever the kernel is asked to change an
> enclave PTE, it needs to ensure the change is consistent with that
> stashed intent.  There is an existing vm_ops->mmap() hook which allows
> SGX to do that for mmap().  However, there is no ->mprotect() hook.  Add
> a vm_ops->mprotect() hook so that mprotect() operations which are
> inconsistent with any page's stashed intent can be rejected by the driver.

Yes to all of the above.

> Implications:
> 
> However, there is currently no implementation of the intent checks at
> the time of ioctl(ENCLAVE_ADD_PAGE).

Correct.

> That means that the intent argument (SGX_PROT_*) is currently unused.

No, the intent argument is used (eventually) by SGX's ->mprotect()
implementation, i.e. sgx_mprotect() enforces that the actual protections are a
subset of the declared/intended protections.

If ->mprotect() is not merged, then it yes, it will be unused.  And therein
lies the problem as the kernel can't start using/enforcing the intent without
breaking userspace.  E.g. an enclave loaded with SGX_PROT_READ but mprotect()'d
with PROT_READ | PROT_EXEC would break if sgx_mprotect() came along.

One way to avoid introducing ->mprotect() would be to require all enclaves to
declare all pages with READ|WRITE|EXEC.  Then we could drop sgx_mprotect()
since the mprotect() permissions are guaranteed to be a subset of the declared
permissions.  That would have the added bonus of eliminating the per-page
checks in sgx_mmap()/sgx_mprotect(), though I've no idea if that is a
meaningful optmization or it's lost in the noise.

The big downside of requiring READ|WRITE|EXEC is that it will make life hell
for a LSM policy owner if they ever want to apply EXECMEM or EXECMOD style
restritions on enclaves, i.e. if SELinux folks want to add
security_enclave_load().

I find that I'm more or less ok with that approach, in no small part because
introducing security_enclave_load() might be a pretty big "if", e.g. security
folks may decide that they'd rather allow/deny enclaves based on the
measurement or signer of the enclave and eschew per-page checks entirely.

> --
> 
> Is that all correct?  Did I miss anything?