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

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Tue, Sep 22, 2020 at 08:35:15AM +0300, Jarkko Sakkinen wrote:
> 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:
> > > 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.

There are scenarios I can contrive, e.g. loading an enclave from a noexec
filesystem without having to copy the entire enclave to anon memory, or
loading a data payload from a noexec FS.

They're definitely contrived scenarios, but given that we also want the
->mprotect() hook/behavior for potential LSM interaction, supporting said
contrived scenarios costs is "free".