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

[Sean Christopherson]

On Tue, Sep 22, 2020 at 12:07:36AM +0300, Jarkko Sakkinen wrote:
> On Mon, Sep 21, 2020 at 09:57:58AM -0700, Sean Christopherson wrote:
> > On Mon, Sep 21, 2020 at 03:49:46PM +0300, Jarkko Sakkinen wrote:
> > > On Fri, Sep 18, 2020 at 04:53:37PM -0700, Sean Christopherson wrote:
> > > > a noexec filesystem by loading code into an enclave, and to give the kernel the
> > > > option of adding enclave specific LSM policies in the future.
> > > > 
> > > > The source file (if one exists) for the enclave is long gone when the enclave
> > > > is actually mmap()'d and mprotect()'d.  To enforce noexec, the requested
> > > > permissions for a given page are snapshotted when the page is added to the
> > > > enclave, i.e. when the enclave is built.  Enclave pages that will be executable
> > > > must originate from an a MAYEXEC VMA, e.g. the source page can't come from a
> > > > noexec file system.
> > > 
> > > noexec check is done in __sgx_encl_add_page(), not in this callback.
> > > sgx_vma_mprotect() calls sgx_encl_may_map(), which iterates the
> > > addresses, checks that permissions are not surpassed and there are
> > > no holes.
> > 
> > Yes, that's what I said.
> 
> sgx_encl_add_page() will remove such page. The callback does not
> interact with this process as such pages never get to the enclave.

I think we're in violent agreement, mostly.

Userspace can add the page without EXEC permissions in the EPCM, and thus
avoid the noexec/VM_MAYEXEC check.  The enclave can then do EMODPE to gain
EXEC permissions in the EPMC.  Without the ->mprotect() hook, we wouldn't
be able to detect/prevent such shenanigans.

> > I would copy-paste the part of the response that was snipped...
> 
> I do agree with the main conclusions but it contains also things that I
> do not see relating that much, like noexec partitions.

As above, this does directly related to noexec/VM_MAYEXEC.

> It goes too far in detail what will LSM's end up doing. I absolutely do not
> want to forecast too far how LSM hooks would work.

That's fine, I was responding to Andy's question, not intending to write a
changelog.
 
> Since we do not have ioctl's for EMODPE and such, I see EMODPE as the
> only reason for doing this right now. Otherwise, we are in trouble with
> any possible LSM callbacks. For any sort of access control decision,
> things decided must stick.

Yes, again, violent agreement :-).

> I would add something like this to the commit message largely based on
> your text:
> 
> "SGX stores the permissions for each page when they are first added, and
> will implement this callback to check that mmap() or mprotect() does not
> surpass these permissions in the requested address range.
> 
> This is done to prevent using EMODPE upgrading permissions of a page
> after mmap() or mprotect() has been done, which would prevent any sort
> of LSM callbacks to be implemented later on because the access control
> decision could deprecate."