Re: [PATCH v3] lockdown,selinux: fix wrong subject in some SELinux lockdown checks

[Ondrej Mosnacek <omosnace@redhat.com>]

On Fri, Jun 18, 2021 at 5:40 AM Paul Moore <paul@paul-moore.com> wrote:
> On Wed, Jun 16, 2021 at 4:51 AM Ondrej Mosnacek <omosnace@redhat.com> wrote:
> >
> > Commit 59438b46471a ("security,lockdown,selinux: implement SELinux
> > lockdown") added an implementation of the locked_down LSM hook to
> > SELinux, with the aim to restrict which domains are allowed to perform
> > operations that would breach lockdown.
> >
> > However, in several places the security_locked_down() hook is called in
> > situations where the current task isn't doing any action that would
> > directly breach lockdown, leading to SELinux checks that are basically
> > bogus.
> >
> > To fix this, add an explicit struct cred pointer argument to
> > security_lockdown() and define NULL as a special value to pass instead
> > of current_cred() in such situations. LSMs that take the subject
> > credentials into account can then fall back to some default or ignore
> > such calls altogether. In the SELinux lockdown hook implementation, use
> > SECINITSID_KERNEL in case the cred argument is NULL.
> >
> > Most of the callers are updated to pass current_cred() as the cred
> > pointer, thus maintaining the same behavior. The following callers are
> > modified to pass NULL as the cred pointer instead:
> > 1. arch/powerpc/xmon/xmon.c
> >      Seems to be some interactive debugging facility. It appears that
> >      the lockdown hook is called from interrupt context here, so it
> >      should be more appropriate to request a global lockdown decision.
> > 2. fs/tracefs/inode.c:tracefs_create_file()
> >      Here the call is used to prevent creating new tracefs entries when
> >      the kernel is locked down. Assumes that locking down is one-way -
> >      i.e. if the hook returns non-zero once, it will never return zero
> >      again, thus no point in creating these files. Also, the hook is
> >      often called by a module's init function when it is loaded by
> >      userspace, where it doesn't make much sense to do a check against
> >      the current task's creds, since the task itself doesn't actually
> >      use the tracing functionality (i.e. doesn't breach lockdown), just
> >      indirectly makes some new tracepoints available to whoever is
> >      authorized to use them.
> > 3. net/xfrm/xfrm_user.c:copy_to_user_*()
> >      Here a cryptographic secret is redacted based on the value returned
> >      from the hook. There are two possible actions that may lead here:
> >      a) A netlink message XFRM_MSG_GETSA with NLM_F_DUMP set - here the
> >         task context is relevant, since the dumped data is sent back to
> >         the current task.
> >      b) When adding/deleting/updating an SA via XFRM_MSG_xxxSA, the
> >         dumped SA is broadcasted to tasks subscribed to XFRM events -
> >         here the current task context is not relevant as it doesn't
> >         represent the tasks that could potentially see the secret.
> >      It doesn't seem worth it to try to keep using the current task's
> >      context in the a) case, since the eventual data leak can be
> >      circumvented anyway via b), plus there is no way for the task to
> >      indicate that it doesn't care about the actual key value, so the
> >      check could generate a lot of "false alert" denials with SELinux.
> >      Thus, let's pass NULL instead of current_cred() here faute de
> >      mieux.
> >
> > Improvements-suggested-by: Casey Schaufler <casey@schaufler-ca.com>
> > Improvements-suggested-by: Paul Moore <paul@paul-moore.com>
> > Fixes: 59438b46471a ("security,lockdown,selinux: implement SELinux lockdown")
> > Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
>
> This seems reasonable to me, but before I merge it into the SELinux
> tree I think it would be good to get some ACKs from the relevant
> subsystem folks.  I don't believe we ever saw a response to the last
> question for the PPC folks, did we?

Can we move this forward somehow, please?

Quoting the yet-unanswered question from the v2 thread for convenience:

> > > The callers migrated to the new hook, passing NULL as cred:
> > > 1. arch/powerpc/xmon/xmon.c
[...]
> >
> > This definitely sounds like kernel_t based on the description above.
>
> Here I'm a little concerned that the hook might be called from some
> unusual interrupt, which is not masked by spin_lock_irqsave()... We
> ran into this with PMI (Platform Management Interrupt) before, see
> commit 5ae5fbd21079 ("powerpc/perf: Fix handling of privilege level
> checks in perf interrupt context"). While I can't see anything that
> would suggest something like this happening here, the whole thing is
> so foreign to me that I'm wary of making assumptions :)
>
> @Michael/PPC devs, can you confirm to us that xmon_is_locked_down() is
> only called from normal syscall/interrupt context (as opposed to
> something tricky like PMI)?

I strongly suspect the answer will be just "Of course it is, why would
you even ask such a silly question?", but please let's have it on
record so we can finally get this patch merged...


> > ---
> >
> > v3:
> > - add the cred argument to security_locked_down() and adapt all callers
> > - keep using current_cred() in BPF, as the hook calls have been shifted
> >   to program load time (commit ff40e51043af ("bpf, lockdown, audit: Fix
> >   buggy SELinux lockdown permission checks"))
> > - in SELinux, don't ignore hook calls where cred == NULL, but use
> >   SECINITSID_KERNEL as the subject instead
> > - update explanations in the commit message
> >
> > v2: https://lore.kernel.org/lkml/20210517092006.803332-1-omosnace@redhat.com/
> > - change to a single hook based on suggestions by Casey Schaufler
> >
> > v1: https://lore.kernel.org/lkml/20210507114048.138933-1-omosnace@redhat.com/
> >
> >  arch/powerpc/xmon/xmon.c             |  4 ++--
> >  arch/x86/kernel/ioport.c             |  4 ++--
> >  arch/x86/kernel/msr.c                |  4 ++--
> >  arch/x86/mm/testmmiotrace.c          |  2 +-
> >  drivers/acpi/acpi_configfs.c         |  2 +-
> >  drivers/acpi/custom_method.c         |  2 +-
> >  drivers/acpi/osl.c                   |  3 ++-
> >  drivers/acpi/tables.c                |  2 +-
> >  drivers/char/mem.c                   |  2 +-
> >  drivers/cxl/mem.c                    |  2 +-
> >  drivers/firmware/efi/efi.c           |  2 +-
> >  drivers/firmware/efi/test/efi_test.c |  2 +-
> >  drivers/pci/pci-sysfs.c              |  6 +++---
> >  drivers/pci/proc.c                   |  6 +++---
> >  drivers/pci/syscall.c                |  2 +-
> >  drivers/pcmcia/cistpl.c              |  2 +-
> >  drivers/tty/serial/serial_core.c     |  2 +-
> >  fs/debugfs/file.c                    |  2 +-
> >  fs/debugfs/inode.c                   |  2 +-
> >  fs/proc/kcore.c                      |  2 +-
> >  fs/tracefs/inode.c                   |  2 +-
> >  include/linux/lsm_hook_defs.h        |  2 +-
> >  include/linux/lsm_hooks.h            |  1 +
> >  include/linux/security.h             |  4 ++--
> >  kernel/bpf/helpers.c                 | 10 ++++++----
> >  kernel/events/core.c                 |  2 +-
> >  kernel/kexec.c                       |  2 +-
> >  kernel/kexec_file.c                  |  2 +-
> >  kernel/module.c                      |  2 +-
> >  kernel/params.c                      |  2 +-
> >  kernel/power/hibernate.c             |  3 ++-
> >  kernel/trace/bpf_trace.c             | 20 ++++++++++++--------
> >  kernel/trace/ftrace.c                |  4 ++--
> >  kernel/trace/ring_buffer.c           |  2 +-
> >  kernel/trace/trace.c                 | 10 +++++-----
> >  kernel/trace/trace_events.c          |  2 +-
> >  kernel/trace/trace_events_hist.c     |  4 ++--
> >  kernel/trace/trace_events_synth.c    |  2 +-
> >  kernel/trace/trace_events_trigger.c  |  2 +-
> >  kernel/trace/trace_kprobe.c          |  6 +++---
> >  kernel/trace/trace_printk.c          |  2 +-
> >  kernel/trace/trace_stack.c           |  2 +-
> >  kernel/trace/trace_stat.c            |  2 +-
> >  kernel/trace/trace_uprobe.c          |  4 ++--
> >  net/xfrm/xfrm_user.c                 | 11 +++++++++--
> >  security/lockdown/lockdown.c         |  3 ++-
> >  security/security.c                  |  4 ++--
> >  security/selinux/hooks.c             |  7 +++++--
> >  48 files changed, 97 insertions(+), 77 deletions(-)
>
> --
> paul moore
> www.paul-moore.com
>

--
Ondrej Mosnacek
Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc