[PATCH v2 0/3] Allow initializing the kernfs node's secctx based on its parent

[Stephen Smalley]

On 1/9/19 12:19 PM, Casey Schaufler wrote:
> On 1/9/2019 8:28 AM, Ondrej Mosnacek wrote:
>> 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/
>>
>> 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 kernfs nodes initially do not store any security context and rely on
>> the LSM to assign some default context to inodes created over them.
> 
> This seems like a bug in kernfs. Why doesn't kernfs adhere to the usual
> and expected filesystem behavior?

sysfs / kernfs didn't support xattrs at all when we first added support 
for setting security contexts to it, so originally all sysfs / kernfs 
inodes had a single security context, and we only required separate 
storage for the inodes that were explicitly labeled by userspace.

Later kernfs grew support for trusted.* xattrs using simple_xattrs but 
the existing security.* support was left mostly unchanged.

> 
>> 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).
> 
> An LSM might use information about the parent other than the "context".
> Smack, for example, uses an attribute SMACK64TRANSMUTE from the parent
> to determine whether the Smack label of the new object should be taken
> from the parent or the process. Passing the "context" of the parent is
> insufficient for Smack.

IIUC, this would involve switching the handling of security.* xattrs in 
kernfs over to use simple_xattrs too (so that we can store multiple such 
attributes), and then pass the entire simple_xattrs list or at least 
anything with a security.* prefix when initializing a new node or 
refreshing an existing inode.  Then the security module could extract 
any security.* attributes of interest for use in determining the label 
of new inodes and in refreshing the label of an inode.

> 
>> 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   | 30 +++++++++++++++++++++++
>>   include/linux/security.h    | 14 +++++++++++
>>   security/security.c         | 10 ++++++++
>>   security/selinux/hooks.c    | 41 +++++++++++++++++++++++++++++++
>>   7 files changed, 149 insertions(+), 8 deletions(-)
>>
>