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

Wed Mar 6 15:54:07 UTC 2019

On Fri, Feb 22, 2019 at 3:57 PM Ondrej Mosnacek <omosnace at redhat.com> wrote:
> 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

Ping about this series... Casey, are you OK with this new version?

-- 
Ondrej Mosnacek <omosnace at redhat dot com>
Associate Software Engineer, Security Technologies
Red Hat, Inc.