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

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Date:   Mon, 10 Jun 2019 09:44:55 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Cedric Xing <cedric.xing@...el.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        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 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.