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From:   Jann Horn <jannh@google.com>
Date:   Wed, 12 Feb 2020 17:54:35 +0100
Message-ID: <CAG48ez0ogRxvCK1aCnviN+nBqp6gmbUD7NjaMKvA7bF=esAc1A@mail.gmail.com>
Subject: Re: [PATCH v2 0/6] Harden userfaultfd
To:     Kees Cook <keescook@chromium.org>
Cc:     Daniel Colascione <dancol@google.com>,
        Tim Murray <timmurray@google.com>,
        Nosh Minwalla <nosh@google.com>,
        Nick Kralevich <nnk@google.com>,
        Lokesh Gidra <lokeshgidra@google.com>,
        kernel list <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        SElinux list <selinux@vger.kernel.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Peter Xu <peterx@redhat.com>,
        linux-security-module <linux-security-module@vger.kernel.org>
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On Wed, Feb 12, 2020 at 8:51 AM Kees Cook <keescook@chromium.org> wrote:
> On Tue, Feb 11, 2020 at 02:55:41PM -0800, Daniel Colascione wrote:
> >   Let userfaultfd opt out of handling kernel-mode faults
> >   Add a new sysctl for limiting userfaultfd to user mode faults
>
> Now this I'm very interested in. Can you go into more detail about two
> things:
[...]
> - Why is this needed in addition to the existing vm.unprivileged_userfaultfd
>   sysctl? (And should this maybe just be another setting for that
>   sysctl, like "2"?)
>
> As to the mechanics of the change, I'm not sure I like the idea of adding
> a UAPI flag for this. Why not just retain the permission check done at
> open() and if kernelmode faults aren't allowed, ignore them? This would
> require no changes to existing programs and gains the desired defense.
> (And, I think, the sysctl value could be bumped to "2" as that's a
> better default state -- does qemu actually need kernelmode traps?)

I think this might be necessary for I/O emulation? As in, if before
getting migrated, the guest writes some data into a buffer, then the
guest gets migrated, and then while the postcopy migration stuff is
still running, the guest tells QEMU to write that data from
guest-physical memory to disk or whatever; I think in that case, QEMU
will do something like a pwrite() syscall where the userspace pointer
points into the memory area containing guest-physical memory, which
would return -EFAULT if userfaultfd was restricted to userspace
accesses.

This was described in this old presentation about why userfaultfd is
better than a SIGSEGV handler:
https://drive.google.com/file/d/0BzyAwvVlQckeSzlCSDFmRHVybzQ/view
(slide 6) (recording at https://youtu.be/pC8cWWRVSPw?t=463)