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

Haitao Huang
2020-09-24T19:12:02Z

On Wed, 23 Sep 2020 08:50:56 -0500, Jarkko Sakkinen
<jarkko.sakkinen@linux.intel.com> 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?