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

[Jarkko Sakkinen]

On Tue, Sep 22, 2020 at 08:30:06AM +0300, Jarkko Sakkinen wrote:
> On Mon, Sep 21, 2020 at 02:18:49PM -0700, Sean Christopherson wrote:
> > 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.
> 
> Right, the VM_MAYEXEC in the code is nested under VM_EXEC check.
> 
> I'm only wondering why not block noexec completely with any permissions,
> i.e. why not just have unconditional VM_MAYEXEC check?

I.e. why not this:

static int __sgx_encl_add_page(struct sgx_encl *encl,
			       struct sgx_encl_page *encl_page,
			       struct sgx_epc_page *epc_page,
			       struct sgx_secinfo *secinfo, unsigned long src)
{
	struct sgx_pageinfo pginfo;
	struct vm_area_struct *vma;
	struct page *src_page;
	int ret;

	vma = find_vma(current->mm, src);
	if (!vma)
		return -EFAULT;

	if (!(vma->vm_flags & VM_MAYEXEC))
		return -EACCES;

I'm not seeing the reason for "partial support" for noexec partitions.

If there is a good reason, fine, let's just then document it.

/Jarkko