[PATCH 6/7] idmapped-mounts: add nested userns creation helpers

Christian Brauner
2021-05-07T15:01:30Z

Add a helper to create a nested userns hierarchy. This will be used in
follow-up tests.

Cc: fstests@vger.kernel.org
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
src/idmapped-mounts/idmapped-mounts.c | 14 ---
src/idmapped-mounts/mount-idmapped.c | 32 +-----
src/idmapped-mounts/utils.c | 160 +++++++++++++++++++++++++-
src/idmapped-mounts/utils.h | 25 ++++
4 files changed, 185 insertions(+), 46 deletions(-)

diff --git a/src/idmapped-mounts/idmapped-mounts.c b/src/idmapped-mounts/idmapped-mounts.c
index 94b83c01..f5a48af7 100644
--- a/src/idmapped-mounts/idmapped-mounts.c
+++ b/src/idmapped-mounts/idmapped-mounts.c
@@ -390,20 +390,6 @@ static inline bool switch_fsids(uid_t fsuid, gid_t fsgid)
return true;
}

-static inline bool switch_ids(uid_t uid, gid_t gid)
-{
- if (setgroups(0, NULL))
- return log_errno(false, "failure: setgroups");
-
- if (setresgid(gid, gid, gid))
- return log_errno(false, "failure: setresgid");
-
- if (setresuid(uid, uid, uid))
- return log_errno(false, "failure: setresuid");
-
- return true;
-}
-
static inline bool switch_userns(int fd, uid_t uid, gid_t gid, bool drop_caps)
{
if (setns(fd, CLONE_NEWUSER))
diff --git a/src/idmapped-mounts/mount-idmapped.c b/src/idmapped-mounts/mount-idmapped.c
index ee3b4f05..46d110fc 100644
--- a/src/idmapped-mounts/mount-idmapped.c
+++ b/src/idmapped-mounts/mount-idmapped.c
@@ -33,36 +33,6 @@

static struct list active_map;

-static int add_map_entry(__u32 id_host,
- __u32 id_ns,
- __u32 range,
- idmap_type_t map_type)
-{
- struct list *new_list = NULL;
- struct id_map *newmap = NULL;
-
- newmap = malloc(sizeof(*newmap));
- if (!newmap)
- return -ENOMEM;
-
- new_list = malloc(sizeof(struct list));
- if (!new_list) {
- free(newmap);
- return -ENOMEM;
- }
-
- *newmap = (struct id_map){
- .hostid = id_host,
- .nsid = id_ns,
- .range = range,
- .map_type = map_type,
- };
-
- new_list->elem = newmap;
- list_add_tail(&active_map, new_list);
- return 0;
-}
-
static int parse_map(char *map)
{
char types[2] = {'u', 'g'};
@@ -91,7 +61,7 @@ static int parse_map(char *map)
else
map_type = ID_TYPE_GID;

- ret = add_map_entry(id_host, id_ns, range, map_type);
+ ret = add_map_entry(&active_map, id_host, id_ns, range, map_type);
if (ret < 0)
return ret;
}
diff --git a/src/idmapped-mounts/utils.c b/src/idmapped-mounts/utils.c
index e54f481d..6ffd6a23 100644
--- a/src/idmapped-mounts/utils.c
+++ b/src/idmapped-mounts/utils.c
@@ -3,11 +3,15 @@
#define _GNU_SOURCE
#endif
#include <fcntl.h>
+#include <grp.h>
#include <linux/limits.h>
+#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
-#include <sched.h>
+#include <sys/eventfd.h>
#include <sys/mount.h>
+#include <sys/prctl.h>
+#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/wait.h>
@@ -137,6 +141,9 @@ static int map_ids_from_idmap(struct list *idmap, pid_t pid)
char mapbuf[4096] = {};
bool had_entry = false;

+ if (list_empty(idmap))
+ return 0;
+
for (idmap_type_t map_type = ID_TYPE_UID, u_or_g = 'u';
map_type <= ID_TYPE_GID; map_type++, u_or_g = 'g') {
char *pos = mapbuf;
@@ -225,3 +232,154 @@ int get_userns_fd(unsigned long nsid, unsigned long hostid, unsigned long range)

return get_userns_fd_from_idmap(&head);
}
+
+bool switch_ids(uid_t uid, gid_t gid)
+{
+ if (setgroups(0, NULL))
+ return syserror("failure: setgroups");
+
+ if (setresgid(gid, gid, gid))
+ return syserror("failure: setresgid");
+
+ if (setresuid(uid, uid, uid))
+ return syserror("failure: setresuid");
+
+ return true;
+}
+
+static int userns_fd_cb(void *data)
+{
+ struct userns_hierarchy *h = data;
+ char c;
+ int ret;
+
+ ret = read_nointr(h->fd_event, &c, 1);
+ if (ret < 0)
+ return syserror("failure: read from socketpair");
+
+ /* Only switch ids if someone actually wrote a mapping for us. */
+ if (c == '1') {
+ if (!switch_ids(0, 0))
+ return syserror("failure: switch ids to 0");
+
+ /* Ensure we can access proc files from processes we can ptrace. */
+ ret = prctl(PR_SET_DUMPABLE, 1, 0, 0, 0);
+ if (ret < 0)
+ return syserror("failure: make dumpable");
+ }
+
+ ret = write_nointr(h->fd_event, "1", 1);
+ if (ret < 0)
+ return syserror("failure: write to socketpair");
+
+ ret = create_userns_hierarchy(++h);
+ if (ret < 0)
+ return syserror("failure: userns level %d", h->level);
+
+ return 0;
+}
+
+int create_userns_hierarchy(struct userns_hierarchy *h)
+{
+ int fret = -1;
+ char c;
+ int fd_socket[2];
+ int fd_userns = -EBADF, ret = -1;
+ ssize_t bytes;
+ pid_t pid;
+ char path[256];
+
+ if (h->level == MAX_USERNS_LEVEL)
+ return 0;
+
+ ret = socketpair(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0, fd_socket);
+ if (ret < 0)
+ return syserror("failure: create socketpair");
+
+ /* Note the CLONE_FILES | CLONE_VM when mucking with fds and memory. */
+ h->fd_event = fd_socket[1];
+ pid = do_clone(userns_fd_cb, h, CLONE_NEWUSER | CLONE_FILES | CLONE_VM);
+ if (pid < 0) {
+ syserror("failure: userns level %d", h->level);
+ goto out_close;
+ }
+
+ ret = map_ids_from_idmap(&h->id_map, pid);
+ if (ret < 0) {
+ kill(pid, SIGKILL);
+ syserror("failure: writing id mapping for userns level %d for %d", h->level, pid);
+ goto out_wait;
+ }
+
+ if (!list_empty(&h->id_map))
+ bytes = write_nointr(fd_socket[0], "1", 1); /* Inform the child we wrote a mapping. */
+ else
+ bytes = write_nointr(fd_socket[0], "0", 1); /* Inform the child we didn't write a mapping. */
+ if (bytes < 0) {
+ kill(pid, SIGKILL);
+ syserror("failure: write to socketpair");
+ goto out_wait;
+ }
+
+ /* Wait for child to set*id() and become dumpable. */
+ bytes = read_nointr(fd_socket[0], &c, 1);
+ if (bytes < 0) {
+ kill(pid, SIGKILL);
+ syserror("failure: read from socketpair");
+ goto out_wait;
+ }
+
+ snprintf(path, sizeof(path), "/proc/%d/ns/user", pid);
+ fd_userns = open(path, O_RDONLY | O_CLOEXEC);
+ if (fd_userns < 0) {
+ kill(pid, SIGKILL);
+ syserror("failure: open userns level %d for %d", h->level, pid);
+ goto out_wait;
+ }
+
+ fret = 0;
+
+out_wait:
+ if (!wait_for_pid(pid) && !fret) {
+ h->fd_userns = fd_userns;
+ fd_userns = -EBADF;
+ }
+
+out_close:
+ if (fd_userns >= 0)
+ close(fd_userns);
+ close(fd_socket[0]);
+ close(fd_socket[1]);
+ return fret;
+}
+
+int add_map_entry(struct list *head,
+ __u32 id_host,
+ __u32 id_ns,
+ __u32 range,
+ idmap_type_t map_type)
+{
+ struct list *new_list = NULL;
+ struct id_map *newmap = NULL;
+
+ newmap = malloc(sizeof(*newmap));
+ if (!newmap)
+ return -ENOMEM;
+
+ new_list = malloc(sizeof(struct list));
+ if (!new_list) {
+ free(newmap);
+ return -ENOMEM;
+ }
+
+ *newmap = (struct id_map){
+ .hostid = id_host,
+ .nsid = id_ns,
+ .range = range,
+ .map_type = map_type,
+ };
+
+ new_list->elem = newmap;
+ list_add_tail(head, new_list);
+ return 0;
+}
diff --git a/src/idmapped-mounts/utils.h b/src/idmapped-mounts/utils.h
index 4f976f9f..9694980e 100644
--- a/src/idmapped-mounts/utils.h
+++ b/src/idmapped-mounts/utils.h
@@ -10,6 +10,7 @@
#include <linux/types.h>
#include <sched.h>
#include <signal.h>
+#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
@@ -19,6 +20,9 @@

#include "missing.h"

+/* Maximum number of nested user namespaces in the kernel. */
+#define MAX_USERNS_LEVEL 32
+
/* A few helpful macros. */
#define STRLITERALLEN(x) (sizeof(""x"") - 1)

@@ -63,6 +67,13 @@ struct list {
struct list *prev;
};

+struct userns_hierarchy {
+ int fd_userns;
+ int fd_event;
+ unsigned int level;
+ struct list id_map;
+};
+
#define list_for_each(__iterator, __list) \
for (__iterator = (__list)->next; __iterator != __list; __iterator = __iterator->next)

@@ -90,6 +101,16 @@ static inline void list_add_tail(struct list *head, struct list *list)
__list_add(list, head->prev, head);
}

+static inline void list_del(struct list *list)
+{
+ struct list *next, *prev;
+
+ next = list->next;
+ prev = list->prev;
+ next->prev = prev;
+ prev->next = next;
+}
+
extern pid_t do_clone(int (*fn)(void *), void *arg, int flags);
extern int get_userns_fd(unsigned long nsid, unsigned long hostid,
unsigned long range);
@@ -97,5 +118,9 @@ extern int get_userns_fd_from_idmap(struct list *idmap);
extern ssize_t read_nointr(int fd, void *buf, size_t count);
extern int wait_for_pid(pid_t pid);
extern ssize_t write_nointr(int fd, const void *buf, size_t count);
+extern bool switch_ids(uid_t uid, gid_t gid);
+extern int create_userns_hierarchy(struct userns_hierarchy *h);
+extern int add_map_entry(struct list *head, __u32 id_host, __u32 id_ns,
+ __u32 range, idmap_type_t map_type);

#endif /* __IDMAP_UTILS_H */
--
2.27.0