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. _______________________________________________ Selinux mailing list Selinux@xxxxxxxxxxxxx To unsubscribe, send email to Selinux-leave@xxxxxxxxxxxxx. To get help, send an email containing "help" to Selinux-request@xxxxxxxxxxxxx.
