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From: Jann Horn <jannh@google.com>
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>
Subject: Re: [PATCH v2 0/6] Harden userfaultfd
Date: Wed, 12 Feb 2020 17:54:35 +0100
Message-ID: <CAG48ez0ogRxvCK1aCnviN+nBqp6gmbUD7NjaMKvA7bF=esAc1A@mail.gmail.com> (raw)
In-Reply-To: <202002112332.BE71455@keescook>

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)

  reply index

Thread overview: 80+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-11 22:55 Daniel Colascione
2020-02-11 22:55 ` [PATCH v2 1/6] Add a new flags-accepting interface for anonymous inodes Daniel Colascione
2020-02-12 16:37   ` Stephen Smalley
2020-02-12 17:23     ` Daniel Colascione
2020-02-11 22:55 ` [PATCH v2 2/6] Add a concept of a "secure" anonymous file Daniel Colascione
2020-02-12 16:49   ` Stephen Smalley
2020-02-14 22:13   ` kbuild test robot
2020-02-11 22:55 ` [PATCH v2 3/6] Teach SELinux about a new userfaultfd class Daniel Colascione
2020-02-12 17:05   ` Stephen Smalley
2020-02-12 17:19     ` Daniel Colascione
2020-02-12 18:04       ` Stephen Smalley
2020-02-12 18:59         ` Stephen Smalley
2020-02-12 19:04           ` Daniel Colascione
2020-02-12 19:11             ` Stephen Smalley
2020-02-12 19:13               ` Daniel Colascione
2020-02-12 19:17               ` Stephen Smalley
2020-02-11 22:55 ` [PATCH v2 4/6] Wire UFFD up to SELinux Daniel Colascione
2020-02-11 22:55 ` [PATCH v2 5/6] Let userfaultfd opt out of handling kernel-mode faults Daniel Colascione
2020-02-11 22:55 ` [PATCH v2 6/6] Add a new sysctl for limiting userfaultfd to user mode faults Daniel Colascione
2020-02-11 23:13 ` [PATCH v2 0/6] Harden userfaultfd Casey Schaufler
2020-02-11 23:27   ` Daniel Colascione
2020-02-12 16:09     ` Stephen Smalley
2020-02-21 17:56     ` James Morris
2020-02-12  7:50 ` Kees Cook
2020-02-12 16:54   ` Jann Horn [this message]
2020-02-12 17:14     ` Peter Xu
2020-02-12 19:41       ` Andrea Arcangeli
2020-02-12 20:04         ` Daniel Colascione
2020-02-12 23:41           ` Andrea Arcangeli
2020-02-12 17:12   ` Daniel Colascione
2020-02-14  3:26 ` [PATCH 0/3] SELinux support for anonymous inodes and UFFD Daniel Colascione
2020-02-14  3:26   ` [PATCH 1/3] Add a new LSM-supporting anonymous inode interface Daniel Colascione
2020-02-14  3:26   ` [PATCH 2/3] Teach SELinux about anonymous inodes Daniel Colascione
2020-02-14 16:39     ` Stephen Smalley
2020-02-14 17:21       ` Daniel Colascione
2020-02-14 18:02         ` Stephen Smalley
2020-02-14 18:08           ` Stephen Smalley
2020-02-14 20:24             ` Stephen Smalley
2020-02-14  3:26   ` [PATCH 3/3] Wire UFFD up to SELinux Daniel Colascione
2020-03-25 23:02   ` [PATCH v2 0/3] SELinux support for anonymous inodes and UFFD Daniel Colascione
2020-03-25 23:02   ` [PATCH v2 1/3] Add a new LSM-supporting anonymous inode interface Daniel Colascione
2020-03-26 13:53     ` Stephen Smalley
2020-03-25 23:02   ` [PATCH v2 2/3] Teach SELinux about anonymous inodes Daniel Colascione
2020-03-26 13:58     ` Stephen Smalley
2020-03-26 17:59       ` Daniel Colascione
2020-03-26 17:37     ` Stephen Smalley
2020-03-25 23:02   ` [PATCH v2 3/3] Wire UFFD up to SELinux Daniel Colascione
2020-03-25 23:49     ` Casey Schaufler
2020-03-26 18:14   ` [PATCH v3 0/3] SELinux support for anonymous inodes and UFFD Daniel Colascione
2020-03-26 18:14     ` [PATCH v3 1/3] Add a new LSM-supporting anonymous inode interface Daniel Colascione
2020-03-26 19:00       ` Stephen Smalley
2020-03-26 18:14     ` [PATCH v3 2/3] Teach SELinux about anonymous inodes Daniel Colascione
2020-03-26 19:02       ` Stephen Smalley
2020-03-26 18:14     ` [PATCH v3 3/3] Wire UFFD up to SELinux Daniel Colascione
2020-03-26 20:06     ` [PATCH v4 0/3] SELinux support for anonymous inodes and UFFD Daniel Colascione
2020-03-26 20:06       ` [PATCH v4 1/3] Add a new LSM-supporting anonymous inode interface Daniel Colascione
2020-03-27 13:40         ` Stephen Smalley
2020-03-26 20:06       ` [PATCH v4 2/3] Teach SELinux about anonymous inodes Daniel Colascione
2020-03-27 13:41         ` Stephen Smalley
2020-03-26 20:06       ` [PATCH v4 3/3] Wire UFFD up to SELinux Daniel Colascione
2020-04-01 21:39       ` [PATCH v5 0/3] SELinux support for anonymous inodes and UFFD Daniel Colascione
2020-04-01 21:39         ` [PATCH v5 1/3] Add a new LSM-supporting anonymous inode interface Daniel Colascione
2020-05-07 16:02           ` James Morris
2020-08-04 21:22           ` Eric Biggers
2020-04-01 21:39         ` [PATCH v5 2/3] Teach SELinux about anonymous inodes Daniel Colascione
2020-04-01 21:39         ` [PATCH v5 3/3] Wire UFFD up to SELinux Daniel Colascione
2020-08-04 21:16           ` Eric Biggers
2020-04-13 13:29         ` [PATCH v5 0/3] SELinux support for anonymous inodes and UFFD Daniel Colascione
2020-04-22 16:55           ` James Morris
2020-04-22 17:12             ` Casey Schaufler
2020-04-23 22:24               ` Casey Schaufler
2020-04-27 16:18                 ` Casey Schaufler
2020-04-27 16:48                   ` Stephen Smalley
2020-04-27 17:12                     ` Casey Schaufler
2020-04-29 17:02                     ` Stephen Smalley
2020-04-27 17:15             ` Casey Schaufler
2020-04-27 19:40               ` Stephen Smalley
2020-06-04  3:56         ` James Morris
2020-06-04 18:51           ` Stephen Smalley
2020-06-04 19:24             ` Lokesh Gidra

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