From: Kees Cook <firstname.lastname@example.org> To: Andrea Arcangeli <email@example.com> Cc: "Michael S. Tsirkin" <firstname.lastname@example.org>, Daniel Colascione <email@example.com>, Jonathan Corbet <firstname.lastname@example.org>, Alexander Viro <email@example.com>, Luis Chamberlain <firstname.lastname@example.org>, Iurii Zaikin <email@example.com>, Mauro Carvalho Chehab <firstname.lastname@example.org>, Andrew Morton <email@example.com>, Andy Shevchenko <firstname.lastname@example.org>, Vlastimil Babka <email@example.com>, Mel Gorman <firstname.lastname@example.org>, Sebastian Andrzej Siewior <email@example.com>, Peter Xu <firstname.lastname@example.org>, Mike Rapoport <email@example.com>, Jerome Glisse <firstname.lastname@example.org>, Shaohua Li <email@example.com>, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org Subject: Re: [PATCH 2/2] Add a new sysctl knob: unprivileged_userfaultfd_user_mode_only Date: Wed, 20 May 2020 11:03:39 -0700 [thread overview] Message-ID: <202005200921.2BD5A0ADD@keescook> (raw) In-Reply-To: <20200520045938.GC26186@redhat.com> On Wed, May 20, 2020 at 12:59:38AM -0400, Andrea Arcangeli wrote: > Hello everyone, > > On Fri, May 08, 2020 at 12:54:03PM -0400, Michael S. Tsirkin wrote: > > On Fri, May 08, 2020 at 12:52:34PM -0400, Michael S. Tsirkin wrote: > > > On Wed, Apr 22, 2020 at 05:26:32PM -0700, Daniel Colascione wrote: > > > > This sysctl can be set to either zero or one. When zero (the default) > > > > the system lets all users call userfaultfd with or without > > > > UFFD_USER_MODE_ONLY, modulo other access controls. When > > > > unprivileged_userfaultfd_user_mode_only is set to one, users without > > > > CAP_SYS_PTRACE must pass UFFD_USER_MODE_ONLY to userfaultfd or the API > > > > will fail with EPERM. This facility allows administrators to reduce > > > > the likelihood that an attacker with access to userfaultfd can delay > > > > faulting kernel code to widen timing windows for other exploits. > > > > > > > > Signed-off-by: Daniel Colascione <email@example.com> > > > > > > The approach taken looks like a hard-coded security policy. > > > For example, it won't be possible to set the sysctl knob > > > in question on any sytem running kvm. So this is > > > no good for any general purpose system. Not all systems run unprivileged KVM. :) > > > What's wrong with using a security policy for this instead? > > > > In fact I see the original thread already mentions selinux, > > so it's just a question of making this controllable by > > selinux. > > I agree it'd be preferable if it was not hardcoded, but then this > patchset is also much simpler than the previous controlling it through > selinux.. > > I was thinking, an alternative policy that could control it without > hard-coding it, is a seccomp-bpf filter, then you can drop 2/2 as > well, not just 1/6-4/6. Err, did I miss a separate 6-patch series? I can't find anything on lore. > > If you keep only 1/2, can't seccomp-bpf enforce userfaultfd to be > always called with flags==0x1 without requiring extra modifications in > the kernel? Please no. This is way too much overhead for something that a system owner wants to enforce globally. A sysctl is the correct option here, IMO. If it needs to be a per-userns sysctl, that would be fine too. > Can't you get the feature party with the CAP_SYS_PTRACE capability > too, if you don't wrap those tasks with the ptrace capability under > that seccomp filter? > > As far as I can tell, it's unprecedented to create a flag for a > syscall API, with the only purpose of implementing a seccomp-bpf > filter verifying such flag is set, but then if you want to control it > with LSM it's even more complex than doing it with seccomp-bpf, and it > requires more kernel code too. We could always add 2/2 later, such > possibility won't disappear, in fact we could also add 1/6-4/6 later > too if that is not enough. > > If we could begin by merging only 1/2 from this new series and be done > with the kernel changes, because we offload the rest of the work to > the kernel eBPF JIT, I think it'd be ideal. I'd agree that patch 1 should land, as it appears to be required for any further policy considerations. I'm still a big fan of a sysctl since this is the kind of thing I would absolutely turn on globally for all my systems. -- Kees Cook
next prev parent reply other threads:[~2020-05-20 18:03 UTC|newest] Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-04-23 0:26 [PATCH 0/2] Control over userfaultfd kernel-fault handling Daniel Colascione 2020-04-23 0:26 ` [PATCH 1/2] Add UFFD_USER_MODE_ONLY Daniel Colascione 2020-07-24 14:28 ` Michael S. Tsirkin 2020-07-24 14:46 ` Lokesh Gidra 2020-07-26 10:09 ` Michael S. Tsirkin 2020-04-23 0:26 ` [PATCH 2/2] Add a new sysctl knob: unprivileged_userfaultfd_user_mode_only Daniel Colascione 2020-05-06 19:38 ` Peter Xu 2020-05-07 19:15 ` Jonathan Corbet 2020-05-20 4:06 ` Andrea Arcangeli 2020-05-08 16:52 ` Michael S. Tsirkin 2020-05-08 16:54 ` Michael S. Tsirkin 2020-05-20 4:59 ` Andrea Arcangeli 2020-05-20 18:03 ` Kees Cook [this message] 2020-05-20 19:48 ` Andrea Arcangeli 2020-05-20 19:51 ` Andrea Arcangeli 2020-05-20 20:17 ` Lokesh Gidra 2020-05-20 21:16 ` Andrea Arcangeli 2020-07-17 12:57 ` Jeffrey Vander Stoep 2020-07-23 17:30 ` Lokesh Gidra 2020-07-24 0:13 ` Nick Kralevich 2020-07-24 13:40 ` Michael S. Tsirkin 2020-08-06 0:43 ` Nick Kralevich 2020-08-06 5:44 ` Michael S. Tsirkin 2020-08-17 22:11 ` Lokesh Gidra 2020-09-04 3:34 ` Andrea Arcangeli 2020-09-05 0:36 ` Lokesh Gidra 2020-09-19 18:14 ` Nick Kralevich 2020-07-24 14:01 ` [PATCH 0/2] Control over userfaultfd kernel-fault handling Michael S. Tsirkin 2020-07-24 14:41 ` Lokesh Gidra
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