On Mon, Jun 12, 2023 at 5:01 AM Ondrej Mosnacek <omosnace@xxxxxxxxxx> wrote: > > Currently, SELinux doesn't allow distinguishing between kernel threads > and userspace processes that are started before the policy is first > loaded - both get the label corresponding to the kernel SID. The only > way a process that persists from early boot can get a meaningful label > is by doing a voluntary dyntransition or re-executing itself. > > Reusing the kernel label for userspace processes is problematic for > several reasons: > 1. The kernel is considered to be a privileged domain and generally > needs to have a wide range of permissions allowed to work correctly, > which prevents the policy writer from effectively hardening against > early boot processes that might remain running unintentionally after > the policy is loaded (they represent a potential extra attack surface > that should be mitigated). > 2. Despite the kernel being treated as a privileged domain, the policy > writer may want to impose certain special limitations on kernel > threads that may conflict with the requirements of intentional early > boot processes. For example, it is a good hardening practice to limit > what executables the kernel can execute as usermode helpers and to > confine the resulting usermode helper processes. However, a > (legitimate) process surviving from early boot may need to execute a > different set of executables. > 3. As currently implemented, overlayfs remembers the security context of > the process that created an overlayfs mount and uses it to bound > subsequent operations on files using this context. If an overlayfs > mount is created before the SELinux policy is loaded, these "mounter" > checks are made against the kernel context, which may clash with > restrictions on the kernel domain (see 2.). > > To resolve this, introduce a new initial SID (reusing the slot of the > former "init" initial SID) that will be assigned to any userspace > process started before the policy is first loaded. This is easy to do, > as we can simply label any process that goes through the > bprm_creds_for_exec LSM hook with the new init-SID instead of > propagating the kernel SID from the parent. > > To provide backwards compatibility for existing policies that are > unaware of this new semantic of the "init" initial SID, introduce a new > policy capability "userspace_initial_context" and set the "init" SID to > the same context as the "kernel" SID unless this capability is set by > the policy. > > Signed-off-by: Ondrej Mosnacek <omosnace@xxxxxxxxxx> > --- > security/selinux/hooks.c | 27 +++++++++++++++++++ > .../selinux/include/initial_sid_to_string.h | 2 +- > security/selinux/include/policycap.h | 1 + > security/selinux/include/policycap_names.h | 3 ++- > security/selinux/include/security.h | 7 +++++ > security/selinux/ss/policydb.c | 27 +++++++++++++++++++ > 6 files changed, 65 insertions(+), 2 deletions(-) Unfortunately we had to revert this due to compatibility issues, but I was hoping there might be a new, fixed version by now; any updates Ondrej? -- paul-moore.com
