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

[Ondrej Mosnacek <omosnace@redhat.com>]

On Sat, Jun 19, 2021 at 12:18 AM Dan Williams <dan.j.williams@intel.com> wrote:
> On Wed, Jun 16, 2021 at 1:51 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>
> [..]
> > diff --git a/drivers/cxl/mem.c b/drivers/cxl/mem.c
> > index 2acc6173da36..c1747b6555c7 100644
> > --- a/drivers/cxl/mem.c
> > +++ b/drivers/cxl/mem.c
> > @@ -568,7 +568,7 @@ static bool cxl_mem_raw_command_allowed(u16 opcode)
> >         if (!IS_ENABLED(CONFIG_CXL_MEM_RAW_COMMANDS))
> >                 return false;
> >
> > -       if (security_locked_down(LOCKDOWN_NONE))
> > +       if (security_locked_down(current_cred(), LOCKDOWN_NONE))
>
> Acked-by: Dan Williams <dan.j.williams@intel.com>
>
> ...however that usage looks wrong. The expectation is that if kernel
> integrity protections are enabled then raw command access should be
> disabled. So I think that should be equivalent to LOCKDOWN_PCI_ACCESS
> in terms of the command capabilities to filter.

Yes, the LOCKDOWN_NONE seems wrong here... but it's a pre-existing bug
and I didn't want to go down yet another rabbit hole trying to fix it.
I'll look at this again once this patch is settled - it may indeed be
as simple as replacing LOCKDOWN_NONE with LOCKDOWN_PCI_ACCESS.

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