[PATCH v7 0/7] Allow initializing the kernfs node's secctx based on its parent

[Ondrej Mosnacek]

TL;DR:
This series adds a new security hook that allows to initialize the security
context of kernfs properly, taking into account the parent context (and
possibly other attributes). 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.

Changes in v7:
- simplify the new security hook's interface
  - rather than trying to extract kernfs data into common structures, just
    pass the kernfs nodes themselves and add helper functions to
    <linux/kernfs.h> for accessing their security xattrs
  - in case other LSMs need more kernfs node attributes than the file mode
    (uid/gid/...), they can simply add new helpers to <linux/kernfs.h> as
    needed
- refactor "kernfs: use simple_xattrs for security attributes" to keep using
  a single common simple_xattrs structure
  - turns out having two separate simple_xattrs wouldn't work right (see
    the definition of simple_xattr_list() in fs/xattr.c)
- drop unnecessary initializations from inode_doinit_use_xattr()
- move the IOP_XATTR check out of inode_doinit_use_xattr()
- add two kernfs cleanup patches
  - these could be applied independently, but the rest of the patches depend on
    them, so I'd rather they stay bundled with the rest to avoid cross-tree
    conflicts

v6: https://lore.kernel.org/selinux/20190214095015.16032-1-omosnace@redhat.com/T/
Changes in v6:
- remove copy-pasted duplicate macro definition

v5: https://lore.kernel.org/selinux/20190205110638.30782-1-omosnace@redhat.com/T/
Changes in v5:
- fix misplaced semicolon detected by 0day robot

v4: https://lore.kernel.org/selinux/20190205085915.5183-1-omosnace@redhat.com/T/
Changes in v4:
- reorder and rename hook arguments
- avoid allocating kernfs_iattrs unless needed

v3: https://lore.kernel.org/selinux/20190130114150.27807-1-omosnace@redhat.com/T/
Changes in v3:
- rename the hook to "kernfs_init_security"
- change the hook interface to simply pass pointers to struct iattr and
  struct simple_xattrs of both the new node and its parent
- add full security xattr support to kernfs (and fixup SELinux behavior
  to handle it properly)

v2: https://lore.kernel.org/selinux/20190109162830.8309-1-omosnace@redhat.com/T/
Changes in v2:
- add docstring for the new hook in union security_list_options
- initialize *ctx to NULL and *ctxlen to 0 in case the hook is not
  implemented

v1: https://lore.kernel.org/selinux/20190109091028.24485-1-omosnace@redhat.com/T/

The kernfs 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
internal structure.

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 can exist for them yet.

Therefore, to allow maintaining similar behavior for kernfs nodes, a new
LSM hook is needed, which will allow initializing the kernfs node's
security context based on its own attributes and those of the parent's
node.

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.

Patch 1/7 simplifies the kernfs_iattrs structure and patch 2/7 optimizes
kernfs to not allocate kernfs_iattrs when getting the value of an xattr.

Patch 3/7 changes SELinux to fetch security context from extended
attributes on kernfs filesystems, falling back to genfs-defined context
if that fails. Without this patch the 4/7 would be a regression for
SELinux (due to the removal of ...notifysecctx() call.

Patch 4/7 implements full security xattr support in kernfs using
simple_xattrs; patch 5/7 adds the new LSM hook; patch 6/7 implements the
new hook in SELinux; and patch 7/7 modifies kernfs to call the new hook
on new node creation.

Testing:
- passed the reproducer from the commit message of the last patch
- passed SELinux testsuite on Fedora Rawhide (x86_64) when applied on top
  of current Rawhide kernel (5.0.0-0.rc7.git2.1) [3]
  - including the new proposed selinux-testsuite subtest [4] (adapted
    from the reproducer)

[1] https://github.com/SELinuxProject/selinux-kernel/issues/39
[2] https://bugzilla.redhat.com/show_bug.cgi?id=1553803
[3] https://koji.fedoraproject.org/koji/taskinfo?taskID=32963825
[4] https://github.com/SELinuxProject/selinux-testsuite/pull/48

Ondrej Mosnacek (7):
  kernfs: clean up struct kernfs_iattrs
  kernfs: do not alloc iattrs in kernfs_xattr_get
  selinux: try security xattr after genfs for kernfs filesystems
  kernfs: use simple_xattrs for security attributes
  LSM: add new hook for kernfs node initialization
  selinux: implement the kernfs_init_security hook
  kernfs: initialize security of newly created nodes

 fs/kernfs/dir.c                     |  28 ++--
 fs/kernfs/inode.c                   | 166 +++++++++------------
 fs/kernfs/kernfs-internal.h         |   8 +-
 fs/kernfs/symlink.c                 |   4 +-
 include/linux/kernfs.h              |  15 ++
 include/linux/lsm_hooks.h           |  13 ++
 include/linux/security.h            |   9 ++
 security/security.c                 |   6 +
 security/selinux/hooks.c            | 223 +++++++++++++++++++---------
 security/selinux/include/security.h |   1 +
 10 files changed, 290 insertions(+), 183 deletions(-)

-- 
2.20.1