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

[Haitao Huang]

On Wed, 23 Sep 2020 08:50:56 -0500, Jarkko Sakkinen  
<jarkko.sakkinen@xxxxxxxxxxxxxxx> wrote:

> On Tue, Sep 22, 2020 at 09:43:02AM -0700, Sean Christopherson wrote:
> > 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".
>
> For me this has caused months of confusion and misunderstanding of this
> feature. I only recently realized that "oh, right, we invented this".
>
> They are contrived scenarios enough that they should be considered when
> the workloads hit.
>
> Either we fully support noexec or not at all. Any "partial" thing is a
> two edged sword: it can bring some robustness with the price of
> complexity and possible unknown uknown scenarios where they might become
> API issue.
>
> I rather think later on how to extend API in some way to enable such
> contrivid scenarios rather than worrying about how this could be abused.
>
> The whole SGX is complex beast already so lets not add any extra when
> there is no a hard requirement to do so.
>
> I'll categorically deny noexec in the next patch set version.
>
> /Jarkko

There are use cases supported currently in which enclave binary is  
received via IPC/RPC and held in buffers before EADD. Denying noexec  
altogether would break those, right?