On 01/08/2016 08:00 AM, Christopher J. PeBenito wrote:
On 1/7/2016 4:19 PM, Stephen Smalley wrote:
On 01/07/2016 03:36 PM, Nicolas Iooss wrote:
Hello,
Since Linux 3.19 targets of /proc/PID/ns/* symlinks have lived in a fs
separated from /proc, named nsfs [1]. These targets are used to enter
the namespace of another process by using setns() syscall [2]. On old
kernels, they were labeled with procfs default type (for example
"getfilecon /proc/self/ns/uts" returned system_u:object_r:proc_t:s0).
When using a recent kernel with a policy without nsfs support, the
inodes are not labeled, as reported for example in Fedora bug #1234757
[3]. As I encounter this issue on my systems, I asked yesterday on the
refpolicy ML how nsfs inodes should be labeled [4].
After digging a little bit about the possibilities, here is a summary of
the options I have considered so far.
Option 1: define a new type to label nsfs inodes, nsfs_t. This works as
expected (c.f. [5] for more details).
Option 2: "fs_use_task nsfs gen_context(system_u:object_r:fs_t,s0);".
Even though this works well for /proc/self/ns/*, this behaves in a weird
way with other processes in the initial namespaces. Here is a shell
session with such a configuration (on a system running in permissive
mode):
# runcon system_u:system_r:init_t sleep 1000000&
[1] 26633
# ls -lZ /proc/26633/ns/uts
lrwxrwxrwx. 1 root root system_u:system_r:init_t 0 Jan 7 19:49
/proc/26633/ns/uts -> uts:[4026531838]
# getfilecon /proc/26633/ns/uts
/proc/26633/ns/uts sysadm_u:sysadm_r:sysadm_t
# runcon user_u:user_r:user_t getfilecon /proc/26633/ns/uts
/proc/26633/ns/uts user_u:user_r:user_t
In short, nsfs inodes get created with the context of the running task.
This is because the inodes do not exist before getfilecon() opens them
(c.f. ns_get_path() function in fs/nsfs.c [6] and
inode_doinit_with_dentry() in security/selinux/hooks.c, which does
"isec->sid = current_sid()" in SECURITY_FS_USE_TASK case [7]). This
issue does not appear with Docker and the network namespace used by
systemd services for PrivateNetwork feature because a file descriptor to
the network namespace is kept open, so the inode is created by the task
"owning" the namespace and its label is stable.
Option 3: do not add anything in the policy and add
"security_task_to_inode(task, inode);" right after the inode
initialization in ns_get_path() (line 90 of [6]), which is what /proc
uses to make /proc/PID inodes have the context of their tasks. Then
"getfilecon /proc/PID/ns/uts" returns the context of task PID (and not
the context that is used by getfilecon command), but as this inode is
per-namespace and not per-task, there can be situations like this:
# id -Z ; getfilecon /proc/self/ns/uts
sysadm_u:sysadm_r:sysadm_t
/proc/self/ns/uts sysadm_u:sysadm_r:sysadm_t
# runcon 'user_u:user_r:user_t' bash -c 'exec 3</proc/self/ns/uts &&
sleep 100000' & P=$!
[1] 803
# ls -lZ "/proc/$P/fd/3"
lr-x------. 1 root root user_u:user_r:user_t 64 Jan 6 23:17
/proc/803/fd/3 -> uts:[4026531838]
# getfilecon "/proc/$P/fd/3" "/proc/$P/ns/uts"
/proc/803/fd/3 user_u:user_r:user_t
/proc/803/ns/uts user_u:user_r:user_t
# getfilecon /proc/self/ns/uts
/proc/self/ns/uts user_u:user_r:user_t
# fg
^C
# getfilecon /proc/self/ns/uts
/proc/self/ns/uts sysadm_u:sysadm_r:sysadm_t
So in fact each /proc/self/ns/* symlink target is labeled accordingly to
the first process which opened it. I do not know whether this behaviour
fits with the "real world usage" of namespaces (e.g. with containers) or
can be considered as a side effect which can be ignored.
Option 4 (not tested): add a sid field to struct ns_common and make
every namespace labeled from the process which creates it, maybe with a
type transition mechanism. This would be quite heavy to handle.
How should nsfs be handled in the kernel and in SELinux policy?
Only option 1 makes sense to me.
I don't understand the usage of nsfs which makes this confusing, but why
doesn't option 3 make sense? Since it's under a particular /proc/pid
entry, doesn't it make sense to label the object as the domain's type?
The symlink is under a particular /proc/pid directory, but the target is
per-namespace, not per-process.
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