This series adds a new security hook that allows to initialize the security context of kernfs properly, taking into account the parent context. Kernfs nodes require special handling here, since they are not bound to specific inodes/superblocks, but instead represent the backing tree structure that is used to build the VFS tree when the kernfs tree is mounted. The kernnfs nodes initially do not store any security context and rely on the LSM to assign some default context to inodes created over them. Kernfs inodes, however, allow setting an explicit context via the *setxattr(2) syscalls, in which case the context is stored inside the kernfs node's metadata. SELinux (and possibly other LSMs) initialize the context of newly created FS objects based on the parent object's context (usually the child inherits the parent's context, unless the policy dictates otherwise). This is done by hooking the creation of the new inode corresponding to the newly created file/directory via security_inode_init_security() (most filesystems always create a fresh inode when a new FS object is created). However, kernfs nodes can be created "behind the scenes" while the filesystem is not mounted anywhere and thus no inodes exist. Therefore, to allow maintaining similar behavior for kernfs nodes, a new LSM hook is needed, which would allow initializing the kernfs node's security context based on the context stored in the parent's node (if any). The main motivation for this change is that the userspace users of cgroupfs (which is built on kernfs) expect the usual security context inheritance to work under SELinux (see [1] and [2]). This functionality is required for better confinement of containers under SELinux. The first patch adds the new LSM hook; the second patch implements the hook in SELinux; and the third patch modifies kernfs to use the new hook to initialize the security context of kernfs nodes whenever its parent node has a non-default context set. Note: the patches are based on current selinux/next [3], but they seem to apply cleanly on top of v5.0-rc1 as well. Testing: - passed SELinux testsuite on Fedora 29 (x86_64) when applied on top of current Rawhide kernel (5.0.0-0.rc1.git0.1) [4] - passed the reproducer from the last patch [1] https://github.com/SELinuxProject/selinux-kernel/issues/39 [2] https://bugzilla.redhat.com/show_bug.cgi?id=1553803 [3] https://git.kernel.org/pub/scm/linux/kernel/git/pcmoore/selinux.git/log/?h=selinux-pr-20181224 [4] https://copr.fedorainfracloud.org/coprs/omos/kernel-testing/build/842855/ Ondrej Mosnacek (3): LSM: Add new hook for generic node initialization selinux: Implement the object_init_security hook kernfs: Initialize security of newly created nodes fs/kernfs/dir.c | 49 ++++++++++++++++++++++++++++++++++--- fs/kernfs/inode.c | 9 +++---- fs/kernfs/kernfs-internal.h | 4 +++ include/linux/lsm_hooks.h | 5 ++++ include/linux/security.h | 12 +++++++++ security/security.c | 8 ++++++ security/selinux/hooks.c | 41 +++++++++++++++++++++++++++++++ 7 files changed, 120 insertions(+), 8 deletions(-) -- 2.20.1
