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

Dan Williams dan.j.williams at intel.com
Fri Jun 18 15:18:04 PDT 2021

On Wed, Jun 16, 2021 at 1:51 AM Ondrej Mosnacek <omosnace at 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 at schaufler-ca.com>
> Improvements-suggested-by: Paul Moore <paul at paul-moore.com>
> Fixes: 59438b46471a ("security,lockdown,selinux: implement SELinux lockdown")
> Signed-off-by: Ondrej Mosnacek <omosnace at 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)
>                 return false;
> -       if (security_locked_down(LOCKDOWN_NONE))
> +       if (security_locked_down(current_cred(), LOCKDOWN_NONE))

Acked-by: Dan Williams <dan.j.williams at 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.

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