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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. Another small backwards compatibility measure is needed in security_sid_to_context_core() for before the initial SELinux policy load - see the code comment for explanation. Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com> Reviewed-by: Stephen Smalley <stephen.smalley.work@gmail.com> [PM: edited comments based on feedback/discussion] Signed-off-by: Paul Moore <paul@paul-moore.com> |
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| .. | ||
| audit.h | ||
| avc_ss.h | ||
| avc.h | ||
| classmap.h | ||
| conditional.h | ||
| ibpkey.h | ||
| ima.h | ||
| initial_sid_to_string.h | ||
| netif.h | ||
| netlabel.h | ||
| netnode.h | ||
| netport.h | ||
| objsec.h | ||
| policycap_names.h | ||
| policycap.h | ||
| security.h | ||
| xfrm.h | ||