Message-ID: <20230803.043622-chrome.spruce.spotty.wine-lWgM90I146nT@cyphar.com>
Date:   Thu, 3 Aug 2023 14:39:52 +1000
From:   Aleksa Sarai <cyphar@...har.com>
To:     Jeff Xu <jeffxu@...gle.com>
Cc:     Jeff Xu <jeffxu@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Daniel Verkamp <dverkamp@...omium.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        YueHaibing <yuehaibing@...wei.com>,
        Christian Brauner <brauner@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [RFC PATCH 0/3] memfd: cleanups for vm.memfd_noexec

On 2023-08-02, Jeff Xu <jeffxu@...gle.com> wrote:
> On Wed, Aug 2, 2023 at 2:39 PM Aleksa Sarai <cyphar@...har.com> wrote:
> >
> > On 2023-08-02, Jeff Xu <jeffxu@...omium.org> wrote:
> > > On Tue, Aug 1, 2023 at 6:05 PM Aleksa Sarai <cyphar@...har.com> wrote:
> > > >
> > > This thread is getting longer with different topics, I will try to
> > > respond with trimmed interleaved replies [1]
> > > There are 3 topics (logging/'migration/ratcheting), this response will
> > > be regarding ratcheting.
> >
> > The migration and ratcheting topics are interconnected because the
> > migration issue makes ratcheting an even more severe issue. But I'll
> > respond to each thread separately.
> >
> > > [1] https://www.kernel.org/doc/html/latest/process/submitting-patches.html?highlight=signed%20off#use-trimmed-interleaved-replies-in-email-discussions
> > >
> > > >
> > > > > > > >  * The ratcheting mechanism for vm.memfd_noexec doesn't make sense as a
> > > > > > > >    security mechanism because a CAP_SYS_ADMIN capable user can create
> > > > > > > >    executable binaries in a hidden tmpfs very easily, not to mention the
> > > > > > > >    many other things they can do.
> > > > > > > >
> > > > > > > By further limiting CAP_SYS_ADMIN, an attacker can't modify this
> > > > > > > sysctl even after compromising some system service with high
> > > > > > > privilege, YAMA has the same approach for ptrace_scope=3
> > > > > >
> > > > > > Personally, I also think this behaviour from YAMA is a little goofy too,
> > > > > > but given that it only locks the most extreme setting and there is no
> > > > > > way to get around the most extreme setting, I guess it makes some sense
> > > > > > (not to mention it's an LSM and so there is an argument that it should
> > > > > > be possible to lock out privileged users from modifying it).
> > > > > > There are many other security sysctls, and very few have this behaviour
> > > > > > because it doesn't make much sense in most cases.
> > > > > >
> > > > > > > In addition, this sysctl is pid_name spaced, this means child
> > > > > > > pid_namespace will alway have the same or stricter security setting
> > > > > > > than its parent, this allows admin to maintain a tree like view. If we
> > > > > > > allow the child pid namespace to elevate its setting, then the
> > > > > > > system-wide setting is no longer meaningful.
> > > > > >
> > > > > > "no longer meaningful" is too strong of a statement imho. It is still
> > > > > > useful for constraining non-root processes and presumably ChromeOS
> > > > > > disallows random processes to do CLONE_NEWUSER (otherwise the protection
> > > > > > of this sysctl is pointless) so in practice for ChromeOS there is no
> > > > > > change in the attack surface.
> > > > > >
> > > > > > (FWIW, I think tying this to the user namespace would've made more sense
> > > > > > since this is about privilege restrictions, but that ship has sailed.)
> > > > > >
> > > > > The reason that this sysctl is a PID namespace is that I hope a
> > > > > container and host can have different sysctl values, e.g. host will
> > > > > allow runc's use of X mfd, while a container  doesn't want X mfd. .
> > > > > To clarify what you meant, do you mean this: when a container is in
> > > > > its own pid_namespace, and has "=2", the programs inside the container
> > > > > can still use CLONE_NEWUSER to break out "=2" ?
> > > >
> > > > With the current implementation, this is not possible. My point was that
> > > > even if it were possible to lower the sysctl, ChromeOS presumably
> > > > already blocks the operations that a user would be able to use to create
> > > > a memfd (an unprivileged user cannot CLONE_NEWPID to modify the sysctl
> > > > without CLONE_NEWUSER, which is presumably blocked on ChromeOS due to
> > > > the other security concerns).
> > > >
> > > >
> > > > > > > The code sample shared in this patch set indicates that the attacker
> > > > > > > already has the ability of creating tmpfs and executing complex steps,
> > > > > > > at that point, it doesn't matter if the code execution is from memfd
> > > > > > > or not. For a safe by default system such as ChromeOS, attackers won't
> > > > > > > easily run arbitrary code, memfd is one of the open doors for that, so
> > > > > > > we are disabling executable memfd in ChromeOS. In other words:  if an
> > > > > > > attacker can already execute the arbitrary code as sample given in
> > > > > > > ChromeOS, without using executable memfd,  then memfd is no longer the
> > > > > > > thing we need to worry about, the arbitrary code execution is already
> > > > > > > achieved by the attacker. Even though I use ChromeOS as an example, I
> > > > > > > think the same type of threat model applies to any system that wants
> > > > > > > to disable executable memfd entirely.
> > > > > >
> > > > > > I understand the threat model this sysctl is blocking, my point is that
> > > > > > blocking CAP_SYS_ADMIN from modifying the setting doesn't make sense
> > > > > > from that threat model. An attacker that manages to trick some process
> > > > > > into creating a memfd with an executable payload is not going to be able
> > > > > > to change the sysctl setting (unless there's a confused deputy with
> > > > > > CAP_SYS_ADMIN, in which case you have much bigger issues).
> > > > > >
> > > > > It is the reverse.  An attacker that manages to trick some
> > > > > CAP_SYSADMIN processes into changing this sysctl value (i.e. lower the
> > > > > setting to 0 if no ratcheting), will be able to continue to use mfd as
> > > > > part of the attack chain.
> > > > >  In chromeOS, an attacker that can change sysctl might not necessarily
> > > > > gain full arbitrary code execution already. As I mentioned previously,
> > > > > the main threat model here is to prevent  arbitrary code execution
> > > > > through mfd.  If an attacker already gains arbitrary code execution,
> > > > > at that point, we no longer worry about mfd.
> > > >
> > > > If an attacker can trick a privileged process into writing to arbitrary
> > > > sysctls, the system has much bigger issues than arbitrary (presumably
> > > > unprivileged) code execution. On the other hand, requiring you to reboot
> > > > a server due to a misconfigured sysctl *is* broken.
> > > >
> > > > Again, at the very least, not even allowing capable(CAP_SYS_ADMIN) to
> > > > change the setting is actually broken.
> > > >
> > > > > > If a CAP_SYS_ADMIN-capable user wants to change the sysctl, blocking it
> > > > > > doesn't add any security because that process could create a memfd-like
> > > > > > fd to execute without issues.
> > > > > >What practical attack does this ratcheting
> > > > > > mechanism protect against? (This is a question you can answer with the
> > > > > > YAMA sysctl, but not this one AFAICS.)
> > > > > >
> > > > > > But even if you feel that allowing this in child user namespaces is
> > > > > > unsafe or undesirable, it's absolutely necessary that
> > > > > > capable(CAP_SYS_ADMIN) should be able to un-brick the running system by
> > > > > > changing the sysctl. The alternative is that you need to reboot your
> > > > > > server in order to un-set a sysctl that broke some application you run.
> > > > > >
> > > > >
> > > > > > Also, by the same token, this ratcheting mechanism doesn't make sense
> > > > > > with =1 *at all* because it could break programs in a way that would
> > > > > > require a reboot but it's not a "security setting" (and the YAMA sysctl
> > > > > > mentioned only locks the sysctl at the highest setting).
> > > > > >
> > > > > I think a system should use "=0" when it is unsure about its program's
> > > > > need or not need executable memfd. Technically, it is not that this
> > > > > sysctl breaks the user, but the admin  made the mistake to set the
> > > > > wrong sysctl value, and an admin should know what they are doing for a
> > > > > sysctl. Yes. rebooting increases the steps to undo the mistake, but
> > > > > that could be an incentive for the admin to fully test its programs
> > > > > before turning on this sysctl - and avoid unexpected runtime errors.
> > > >
> > > > I don't think this stance is really acceptable -- if an admin that has
> > > > privileges to load kernel modules is not able to disable a sysctl that
> > > > can break working programs without rebooting there is
> > > >
> > > > When this sysctl was first proposed a few years ago (when kernel folks
> > > > found out that runc was using executable memfds), my understanding is
> > > > that the long-term goal was to switch programs to have
> > > > non-executable-memfds by default on most distributions. Making it
> > > > impossible for an admin to lower the sysctl value flies in the face of
> > > > this goal.
> > > >
> > > > At the very least, being unable to lower the sysctl from =1 to =0 is
> > > > just broken (even if you use the yama example -- yama only locks the
> > > > sysctl at highest possible setting, not on lower settings). But in my
> > > > view, having this sysctl ratchet at all doesn't make sense.
> > > >
> > > To reiterate/summarize the current mechanism for vm.memfd_noexec
> > >
> > > 1> It is a pid namespace sysctl,  init ns and child pid ns can have
> > > different setting values.
> > > 2> child pid ns inherits parent's pid ns's sysctl at the time of fork.
> > > 3> There are  3 values for the sysctl, each higher value is more
> > > restrictive than the lower one. Once set, doesn't allow downgrading.
> > >
> > > It can be used as  following:
> > > 1>
> > > init ns: vm.memfd_noexec = 2 (at boot time)
> > > Not allow executable memfd for the entire system, including its containers.
> > >
> > > 2>
> > > init ns: vm.memfd_noexec = 0 or 1
> > > container (child init namespace) vm.memfd_noexec = 2.
> > > The host allows runc's usage of executable memfd during container
> > > creation. Inside the container, executable memfd is not allowed.
> > >
> > > The inherence + not allow downgrading is to reason with how
> > > vm.memfd_noexec is applied in the process tree.
> > > Without it, essentially we are losing the hierarchy view across the
> > > process tree and  a process can evaluate its capability by modifying
> > > the setting. I think that is a less secure approach I would not
> > > prefer.
> >
> > If you really want the hierarchical aspect, we can implement it so that
> > it's _actually_ hierarchical like so:
> >
> >  * By default, your setting is the same as your parent (this is checked
> >    by going up the pidns tree -- a-la cgroups). This is less efficient
> >    but you want a hierarchy, so we can do it this way instead.
> >  * If you set a specific setting, that takes precedence but only if it's
> >    a greater or equal setting to your parent.
> >  * Trying to set a lower setting than your parent fails regardless of
> >    privileges.
> >
> > This will allow *privileged users* to lower the setting, but only if
> > the parent pidns also has a lower setting. This allows a system admin to
> > enforce the setting. It seems to me that this fulfils all the
> > requirements you have.
> >
> > Most importantly, this would allow for a hierarchical view without
> > having a sysctl that will break systems and nobody will use. I need to
> > re-iterate this point -- nobody is going to use this sysctl as it
> > currently works because it ratchets in a way that admins cannot undo. In
> > practice this would mean you would need to reboot your whole datacenter
> > if you didn't catch that an update to one of you dependencies didn't
> > pass a required *noop* flag to memfd_create().
> >
> Yes. I agree this is another way to implement a hierarchical view,
> which is a little more costly,  because it goes up the process tree.

The maximum height is 32 namespaces, it's not that bad. I'm working on a
v2 that implements it this way instead.

I also just figured out that there is a flaw with the current
implementation's "hierarchy" -- if you create a pid namespace and then
change vm.memfd_noexec, the limit doesn't apply to the child pidns. This
makes sense given the implementation, but it means that the "tree view"
you talked about doesn't actually apply to your implementation.

> I respectfully disagree that nobody will use the current sysctl
> though, I can still see that a container might want this,  e.g. a
> small container that doesn't require a lot of refactoring to add NX,
> and restarting container usually isn't a problem, and container might
> like the fact that downgrade is denied at run time.

I should've specified that I was talking about using the setting on the
host (which is presumably the goal -- at the very least I presume the
goal is to get distributions to use =1 at some point).

Also, funnily enough, container runtimes use executable memfds. :P

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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