[RFC PATCH 3/8] kmod - teach call_usermodehelper() to use a namespace

[Ian Kent]

The call_usermodehelper() function executes all binaries in the
global "init" root context. This doesn't allow a binary to be run
within a namespace (eg. the namespace of a container).

Both containerized NFS client and NFS server need the ability to
execute a binary in a container's context. To do this use the init
process of the callers environment is used to setup the namespaces
in the same way the root init process is used otherwise.

Signed-off-by: Ian Kent <ikent@xxxxxxxxxx>
Cc: Benjamin Coddington <bcodding@xxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: J. Bruce Fields <bfields@xxxxxxxxxxxx>
Cc: David Howells <dhowells@xxxxxxxxxx>
Cc: Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <onestero@xxxxxxxxxx>
Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
Cc: Jeff Layton <jeff.layton@xxxxxxxxxxxxxxx>
---
 include/linux/kmod.h |   16 +++++++
 kernel/kmod.c        |  115 +++++++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 128 insertions(+), 3 deletions(-)

diff --git a/include/linux/kmod.h b/include/linux/kmod.h
index 15bdeed..b0f1b3c 100644
--- a/include/linux/kmod.h
+++ b/include/linux/kmod.h
@@ -52,6 +52,7 @@ struct file;
 #define UMH_WAIT_EXEC	1	/* wait for the exec, but not the process */
 #define UMH_WAIT_PROC	2	/* wait for the process to complete */
 #define UMH_KILLABLE	4	/* wait for EXEC/PROC killable */
+#define UMH_USE_NS	8	/* exec using caller's init namespace */
 
 struct subprocess_info {
 	struct work_struct work;
@@ -69,6 +70,21 @@ struct subprocess_info {
 extern int
 call_usermodehelper(char *path, char **argv, char **envp, int flags);
 
+#if !defined(CONFIG_PROC_FS) || !defined(CONFIG_NAMESPACES)
+inline struct task_struct *umh_get_init_task(void)
+{
+	return ERR_PTR(-ENOTSUP);
+}
+
+inline int umh_enter_ns(struct task_struct *tsk, struct cred *new)
+{
+	return -ENOTSUP;
+}
+#else
+struct task_struct *umh_get_init_pid(void);
+int umh_enter_ns(struct task_struct *tsk, struct cred *new);
+#endif
+
 extern struct subprocess_info *
 call_usermodehelper_setup(char *path, char **argv, char **envp, gfp_t gfp_mask,
 			  int (*init)(struct subprocess_info *info, struct cred *new),
diff --git a/kernel/kmod.c b/kernel/kmod.c
index 14c0188..4c649d6 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -582,6 +582,98 @@ unlock:
 }
 EXPORT_SYMBOL(call_usermodehelper_exec);
 
+#if defined(CONFIG_PROC_FS) && defined(CONFIG_NAMESPACES)
+#define NS_PATH_MAX	35
+#define NS_PATH_FMT	"%lu/ns/%s"
+
+/* Note namespace name order is significant */
+static const char *ns_names[] = { "user", "ipc", "uts", "net", "pid", "mnt", NULL };
+
+struct task_struct *umh_get_init_pid(void)
+{
+	struct task_struct *tsk;
+
+	rcu_read_lock();
+	tsk = find_task_by_vpid(1);
+	if (tsk)
+		get_task_struct(tsk);
+	rcu_read_unlock();
+	if (!tsk)
+		return ERR_PTR(-ESRCH);
+
+	return tsk;
+}
+EXPORT_SYMBOL(umh_get_init_pid);
+
+int umh_enter_ns(struct task_struct *tsk, struct cred *new)
+{
+	char path[NS_PATH_MAX];
+	struct vfsmount *mnt;
+	const char *name;
+	pid_t pid;
+	int err = 0;
+
+	pid = task_pid_nr(tsk);
+
+	/*
+	 * The user mode thread runner runs in the root init namespace
+	 * so it will see all system pids.
+	 */
+	mnt = task_active_pid_ns(current)->proc_mnt;
+
+	for (name = ns_names[0]; *name; name++) {
+		struct file *this;
+		int len;
+
+		len = snprintf(path,
+			       NS_PATH_MAX, NS_PATH_FMT,
+			       (unsigned long) pid, name);
+		if (len >= NS_PATH_MAX) {
+			err = -ENAMETOOLONG;
+			break;
+		}
+
+		this = file_open_root(mnt->mnt_root, mnt, path, O_RDONLY);
+		if (unlikely(IS_ERR(this))) {
+			err = PTR_ERR(this);
+			break;
+		}
+
+		err = setns_inode(file_inode(this), 0);
+		fput(this);
+		if (err)
+			break;
+	}
+
+	return err;
+}
+EXPORT_SYMBOL(umh_enter_ns);
+
+static int umh_set_ns(struct subprocess_info *info, struct cred *new)
+{
+	struct task_struct *tsk = info->data;
+
+	return umh_enter_ns(tsk, new);
+}
+
+static void umh_free_ns(struct subprocess_info *info)
+{
+	struct task_struct *tsk = info->data;
+
+	if (tsk)
+		put_task_struct(tsk);
+}
+#else
+static int umh_set_ns(struct subprocess_info *info, struct cred *new)
+{
+	return 0;
+}
+
+static void umh_free_ns(struct subprocess_info *info)
+{
+}
+#endif
+
 /**
  * call_usermodehelper() - prepare and start a usermode application
  * @path: path to usermode executable
@@ -599,11 +691,28 @@ int call_usermodehelper(char *path, char **argv, char **envp, int flags)
 {
 	struct subprocess_info *info;
 	gfp_t gfp_mask = (flags == UMH_NO_WAIT) ? GFP_ATOMIC : GFP_KERNEL;
+	unsigned int use_ns = flags & UMH_USE_NS;
+	struct task_struct *tsk = NULL;
+
+	if (use_ns) {
+		tsk = umh_get_init_pid();
+		if (IS_ERR(tsk))
+			return PTR_ERR(tsk);
+	}
 
-	info = call_usermodehelper_setup(path, argv, envp, gfp_mask,
-					 NULL, NULL, NULL);
-	if (info == NULL)
+	if (!tsk)
+		info = call_usermodehelper_setup(path, argv, envp,
+						 gfp_mask, NULL, NULL, NULL);
+	else {
+		info = call_usermodehelper_setup(path, argv, envp, gfp_mask,
+						 umh_set_ns, umh_free_ns,
+						 tsk);
+	}
+	if (info == NULL) {
+		if (tsk)
+			put_task_struct(tsk);
 		return -ENOMEM;
+	}
 
 	return call_usermodehelper_exec(info, flags);
 }

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