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


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