On Thu, Sep 23, 2021 at 5:18 PM Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
>
> The original SELinux lockdown implementation in 59438b46471a
> ("security,lockdown,selinux: implement SELinux lockdown") used the
> current task's credentials as both the subject and object in the
> SELinux lockdown hook, selinux_lockdown(). Unfortunately that
> proved to be incorrect in a number of cases as the core kernel was
> calling the LSM lockdown hook in places where the credentials from
> the "current" task_struct were not the correct credentials to use
> in the SELinux access check.
>
> Attempts were made to resolve this by adding a credential pointer
> to the LSM lockdown hook as well as suggesting that the single hook
> be split into two: one for user tasks, one for kernel tasks; however
> neither approach was deemed acceptable by Linus.
>
> In order to resolve the problem of an incorrect SELinux domain being
> used in the lockdown check, this patch makes the decision to perform
> all of the lockdown access control checks against the
> SECINITSID_KERNEL domain. This is far from ideal, but it is what
> we have available to us at this point in time.
>
> Fixes: 59438b46471a ("security,lockdown,selinux: implement SELinux lockdown")
> Signed-off-by: Paul Moore <paul@xxxxxxxxxxxxxx>
>
> --
>
> NOTES: While trivial, this patch is completely untested; I'm posting
> this simply to see what comments there may be within the SELinux
> community to such an approach as the current code is flawed and needs
> to be corrected.
> ---
> security/selinux/hooks.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 6517f221d52c..4f016a49e3a6 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -7016,7 +7016,7 @@ static void selinux_bpf_prog_free(struct bpf_prog_aux *aux)
> static int selinux_lockdown(enum lockdown_reason what)
> {
> struct common_audit_data ad;
> - u32 sid = current_sid();
> + u32 sid = SECINITSID_KERNEL;
> int invalid_reason = (what <= LOCKDOWN_NONE) ||
> (what == LOCKDOWN_INTEGRITY_MAX) ||
> (what >= LOCKDOWN_CONFIDENTIALITY_MAX);
Kind of begs the question of whether it is even worth keeping the check at all.
This could potentially break an existing policy where lockdown has
been defined and only allowed as needed but I suspect it is already
allowed in Fedora and Android.
