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

[Paul Moore <paul@paul-moore.com>]

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?

> ---
>
> 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