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

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Date:   Tue, 11 Jun 2019 13:21:21 -0400
From:   Stephen Smalley <sds@...ho.nsa.gov>
To:     Andy Lutomirski <luto@...nel.org>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Cedric Xing <cedric.xing@...el.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        Paul Moore <paul@...l-moore.com>,
        Eric Paris <eparis@...isplace.org>, selinux@...r.kernel.org,
        Jethro Beekman <jethro@...tanix.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        linux-sgx@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>, nhorman@...hat.com,
        npmccallum@...hat.com, Serge Ayoun <serge.ayoun@...el.com>,
        Shay Katz-zamir <shay.katz-zamir@...el.com>,
        Haitao Huang <haitao.huang@...el.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Kai Svahn <kai.svahn@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        Josh Triplett <josh@...htriplett.org>,
        Kai Huang <kai.huang@...el.com>,
        David Rientjes <rientjes@...gle.com>,
        William Roberts <william.c.roberts@...el.com>,
        Philip Tricca <philip.b.tricca@...el.com>
Subject: Re: [RFC PATCH v2 3/5] x86/sgx: Enforce noexec filesystem restriction
 for enclaves

On 6/10/19 12:44 PM, Andy Lutomirski wrote:
> On Mon, Jun 10, 2019 at 9:00 AM Jarkko Sakkinen
> <jarkko.sakkinen@...ux.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.

What's a situation where we would want to allow this?  Why is it 
different than do_mmap()?