[RFC PATCH v2 3/5] x86/sgx: Enforce noexec filesystem restriction for enclaves

[Andy Lutomirski]

On Mon, Jun 10, 2019 at 9:00 AM Jarkko Sakkinen
<jarkko.sakkinen at linux.intel.com> wrote:
>
> On Wed, Jun 05, 2019 at 07:11:43PM -0700, Sean Christopherson wrote:
> > +             goto out;
> > +     }
> > +
> > +     /*
> > +      * Query VM_MAYEXEC as an indirect path_noexec() check (see do_mmap()),
> > +      * but with some future proofing against other cases that may deny
> > +      * execute permissions.
> > +      */
> > +     if (!(vma->vm_flags & VM_MAYEXEC)) {
> > +             ret = -EACCES;
> > +             goto out;
> > +     }
> > +
> > +     if (copy_from_user(dst, (void __user *)src, PAGE_SIZE))
> > +             ret = -EFAULT;
> > +     else
> > +             ret = 0;
> > +
> > +out:
> > +     up_read(&current->mm->mmap_sem);
> > +
> > +     return ret;
> > +}
>
> I would suggest to express the above instead like this for clarity
> and consistency:
>
>                 goto err_map_sem;
>         }
>
>         /* Query VM_MAYEXEC as an indirect path_noexec() check
>          * (see do_mmap()).
>          */
>         if (!(vma->vm_flags & VM_MAYEXEC)) {
>                 ret = -EACCES;
>                 goto err_mmap_sem;
>         }
>
>         if (copy_from_user(dst, (void __user *)src, PAGE_SIZE)) {
>                 ret = -EFAULT;
>                 goto err_mmap_sem;
>         }
>
>         return 0;
>
> err_mmap_sem:
>         up_read(&current->mm->mmap_sem);
>         return ret;
> }
>
> The comment about future proofing is unnecessary.
>

I'm also torn as to whether this patch is needed at all.  If we ever
get O_MAYEXEC, then enclave loaders should use it to enforce noexec in
userspace.  Otherwise I'm unconvinced it's that special.