Re: [PATCH v4 0/2] Control over userfaultfd kernel-fault handling

[Lokesh Gidra <lokeshgidra@google.com>]

On Thu, Oct 8, 2020 at 4:22 PM Nick Kralevich <nnk@google.com> wrote:
>
> On Wed, Oct 7, 2020 at 9:01 PM Andrea Arcangeli <aarcange@redhat.com> wrote:
> >
> > Hello Lokesh,
> >
> > On Wed, Oct 07, 2020 at 01:26:55PM -0700, Lokesh Gidra wrote:
> > > On Wed, Sep 23, 2020 at 11:56 PM Lokesh Gidra <lokeshgidra@google.com> wrote:
> > > >
> > > > This patch series is split from [1]. The other series enables SELinux
> > > > support for userfaultfd file descriptors so that its creation and
> > > > movement can be controlled.
> > > >
> > > > It has been demonstrated on various occasions that suspending kernel
> > > > code execution for an arbitrary amount of time at any access to
> > > > userspace memory (copy_from_user()/copy_to_user()/...) can be exploited
> > > > to change the intended behavior of the kernel. For instance, handling
> > > > page faults in kernel-mode using userfaultfd has been exploited in [2, 3].
> > > > Likewise, FUSE, which is similar to userfaultfd in this respect, has been
> > > > exploited in [4, 5] for similar outcome.
> > > >
> > > > This small patch series adds a new flag to userfaultfd(2) that allows
> > > > callers to give up the ability to handle kernel-mode faults with the
> > > > resulting UFFD file object. It then adds a 'user-mode only' option to
> > > > the unprivileged_userfaultfd sysctl knob to require unprivileged
> > > > callers to use this new flag.
> > > >
> > > > The purpose of this new interface is to decrease the chance of an
> > > > unprivileged userfaultfd user taking advantage of userfaultfd to
> > > > enhance security vulnerabilities by lengthening the race window in
> > > > kernel code.
> > > >
> > > > [1] https://lore.kernel.org/lkml/20200211225547.235083-1-dancol@google.com/
> > > > [2] https://duasynt.com/blog/linux-kernel-heap-spray
> > > > [3] https://duasynt.com/blog/cve-2016-6187-heap-off-by-one-exploit
> >
> > I've looking at those links and I've been trying to verify the link
> > [3] is relevant.
> >
> > Specifically I've been trying to verify if 1) current state of the art
> > modern SLUB randomization techniques already enabled in production and
> > rightfully wasting some CPU in all enterprise kernels to prevent
> > things like above to become an issue in practice 2) combined with the
> > fact different memcg need to share the same kmemcaches (which was
> > incidentally fixed a few months ago upstream) and 3) further
> > robustness enhancements against exploits in the slub metadata, may
> > already render the exploit [3] from 2016 irrelevant in practice.
>
> It's quite possible that some other mitigation was helpful against the
> technique used by this particular exploit. It's the nature of exploits
> that they are fragile and will change as new soft mitigations are
> introduced. The effectiveness of a particular exploit mitigation
> change is orthogonal to the change presented here.
>
> The purpose of this change is to prevent an attacker from suspending
> kernel code execution and having kernel data structures in a
> predictable state. This makes it harder for an attacker to "win" race
> conditions against various kernel data structures. This change
> compliments other kernel hardening changes such as the changes you've
> referenced above. Focusing on one particular exploit somewhat misses
> the point of this change.
>
> >
> > So I started by trying to reproduce [3] by building 4.5.1 with a
> > .config with no robustness features and I booted it on fedora-32 or
> > gentoo userland and I cannot even invoke call_usermodehelper. Calling
> > socket(22, AF_INET, 0) won't invoke such function. Can you reproduce
> > on 4.5.1? Which kernel .config should I use to build 4.5.1 in order
> > for call_usermodehelper to be invoked by the exploit? Could you help
> > to verify it?
>
> I haven't tried to verify this myself. I wonder if the usermode
> hardening changes also impacted this exploit? See
> https://lkml.org/lkml/2017/1/16/468
>
> But again, focusing on an exploit, which is inherently fragile in
> nature and dependent on the state of the kernel tree at a particular
> time, is unlikely to be useful to analyze this patch.
>
> >
> > It even has uninitialized variable spawning random perrors so it
> > doesn't give a warm fuzzy feeling:
> >
> > ====
> > int main(int argc, char **argv) {
> >         void *region, *map;
> >                       ^^^^^
> >         pthread_t uffd_thread;
> >         int uffd, msqid, i;
> >
> >         region = (void *)mmap((void *)0x40000000, 0x2000, PROT_READ|PROT_WRITE,
> >                                MAP_FIXED|MAP_PRIVATE|MAP_ANON, -1, 0);
> >
> >         if (!region) {
> >                 perror("mmap");
> >                 exit(2);
> >         }
> >
> >         setup_pagefault(region + 0x1000, 0x1000, 1);
> >
> >         printf("my pid = %d\n", getpid());
> >
> >         if (!map) {
> >         ^^^^^^^^
> >                 perror("mmap");
> > ====
> >
> > The whole point of being able to reproduce on 4.5.1 is then to
> > simulate if the same exploit would also reproduce on current kernels
> > with all enterprise default robustness features enabled. Or did
> > anybody already verify it?
> >
> > Anyway the links I was playing with are all in the cover letter, the
> > cover letter is not as important as the actual patches. The actual
> > patches looks fine to me.
>
> That's great to hear.
>
> >
> > The only improvement I can think of is, what about to add a
> > printk_once to suggest to toggle the sysctl if userfaultfd bails out
> > because the process lacks the CAP_SYS_PTRACE capability? That would
> > facilitate the /etc/sysctl.conf or tuned tweaking in case the apps
> > aren't verbose enough.
> >
> > It's not relevant anymore with this latest patchset, but about the
> > previous argument that seccomp couldn't be used in all Android
> > processes because of performance concern, I'm slightly confused.
>
> Seccomp causes more problems than just performance. Seccomp is not
> designed for whole-of-system protections. Please see my other writeup
> at https://lore.kernel.org/lkml/CAFJ0LnEo-7YUvgOhb4pHteuiUW+wPfzqbwXUCGAA35ZMx11A-w@mail.gmail.com/
>
> >
> > https://android-developers.googleblog.com/2017/07/seccomp-filter-in-android-o.html
> >
> > "Android O includes a single seccomp filter installed into zygote, the
> > process from which all the Android applications are derived. Because
> > the filter is installed into zygote—and therefore all apps—the Android
> > security team took extra caution to not break existing apps"
> >
> > Example:
> >
> > $ uname -mo
> > aarch64 Android
> > $ cat swapoff.c
> > #include <sys/swap.h>
> >
> > int main()
> > {
> >         swapoff("");
> > }
> > $ gcc swapoff.c -o swapoff -O2
> > $ ./swapoff
> > Bad system call
> > $
> >
> > It's hard to imagine what is more performance critical than the zygote
> > process and the actual apps as above?
> >
> > It's also hard to imagine what kind of performance concern can arise
> > by adding seccomp filters also to background system apps that
> > generally should consume ~0% of CPU.
> >
> > If performance is really a concern, the BPF JIT representation with
> > the bitmap to be able to run the filter in O(1) sounds a better
> > solution than not adding ad-hoc filters and it's being worked on for
> > x86-64 and can be ported to aarch64 too. Many of the standalone
> > background processes likely wouldn't even use uffd at all so you could
> > block the user initiated faults too that way.
> >
> > Ultimately because of issues as [3] (be them still relevant or not, to
> > be double checked), no matter if through selinux, seccomp or a
> > different sysctl value, without this patchset applied the default
> > behavior of the userfaultfd syscall for all Linux binaries running on
> > Android kernels, would deviate from the upstream kernel. So even if we
> > would make the pipe mutex logic more complex the deviation would
> > remain. Your patchset adds much less risk of breakage than adding a
> > timeout to kernel initiated userfaults and it resolves all concerns as
> > well as a timeout. We'll also make better use of the "0" value this
> > way. So while I'm not certain this is the best for the long term, this
> > looks the sweet spot for the short term to resolve many issues at
> > once.
> >
> > Thanks!
> > Andrea
> >
>
>
> --
> Nick Kralevich | nnk@google.com

Hi Andrea,

Did you get a chance to go through Nick's reply to your questions?
Also, I sent another revision of this patch series which takes care of
the printk that you suggested. Please take a look.