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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 3 Apr 2018 17:08:30 -0700
Message-ID: <CA+55aFzH6doC2F1xBitPRwTZtbiCO_mX+QVcJ8HQCdsdD3Vmpg@mail.gmail.com>
Subject: Re: [GIT PULL] Kernel lockdown for secure boot
To:     Matthew Garrett <mjg59@google.com>
Cc:     Andrew Lutomirski <luto@kernel.org>,
        David Howells <dhowells@redhat.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        James Morris <jmorris@namei.org>,
        Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Justin Forbes <jforbes@redhat.com>,
        linux-man <linux-man@vger.kernel.org>, joeyli <jlee@suse.com>,
        LSM List <linux-security-module@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Kees Cook <keescook@chromium.org>,
        linux-efi <linux-efi@vger.kernel.org>
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On Tue, Apr 3, 2018 at 5:04 PM, Matthew Garrett <mjg59@google.com> wrote:
>
> How? When there are random DMA-capable PCI devices that are driven by
> userland tools that are mmap()ing the BARs out of sysfs, how do we
> simultaneously avoid breaking those devices while also preventing the
> majority of users from being vulnerable to an attacker just DMAing over the
> kernel?

.. if that ends up being a real problem, then you print a warning and
tell people to use the kernel command line to disable things.

And if it's a big and common problem, then the answer may be that
lockdown has to be entirely OFF by default, and you instead just tell
people to enable it manually with a kernel command line option.

Still better than telling them to disable/enable secure boot, which
they may or may not even be able to to.

                 Linus