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

[Stephen Smalley]

On 1/9/19 5:03 PM, Casey Schaufler wrote:
> On 1/9/2019 12:37 PM, Stephen Smalley wrote:
>> 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.
> 
> OK, so as I said, this seems like a bug in kernfs.
> 
>>
>>>
>>>> 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.
> 
> Right. But I'll point out that there is nothing to prevent an
> LSM from using inode information outside of the xattrs (e.g. uids)
> to determine the security state it wants to give a new object.

If that's a real concern, the hook could pass the ia_iattr structure in 
addition to the simple_xattrs list and the security module could use any 
inode attributes it likes in making the decision.  Effectively it would 
be passing the entire kernfs_iattrs structure, but probably not directly 
since that definition is presently private to kernfs.

> I suggest that the better solution would be for kernfs to
> use inodes like a real filesystem. Every special case like this
> results in special cases like this special hook. It's hard
> enough to keep track of the general case in the Linux kernel.

Feel free to propose an implementation if you like, but doing a complete 
rewrite of kernfs internals seems a bit out of scope.

> 
>>
>>>
>>>> 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(-)
>>>>
>>>
>>
>>
>