Hi Eric,
On 25 January 2017 at 14:58, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
> "Michael Kerrisk (man-pages)" <mtk.manpages@xxxxxxxxx> writes:
>
>> I would like to write code that discovers the namespace setup on a live
>> system. The NS_GET_PARENT and NS_GET_USERNS ioctl() operations added in
>> Linux 4.9 provide much of what I want, but there are still a couple of
>> small pieces missing. Those pieces are added with this patch series.
>
> So it looks like the -EOVERFLOW change broke your example program.
> Causing it to abort if -EOVERFLOW is hit. Do we really want to return
> -EOVERFLOW? Or do you want to fix your program?
Bother! Yes, I should have kept the example program in sync. (I
overlooked that it was not any more in sync.)
So, I want to make sure I understand correctly, before I aswer your
question. Suppose we have
1. Outer namespace owned by UID 0
2. Inner namespace owned by UID 1000
3. A UID mapping in the inner namespace that maps '0 1000 1'
4. A processs, X, in the outer namespace with UID 0 (and all caps).
That's the case you're meaning, right? So, UID 0 doesn't have a
mapping into the inner namespace, but does have all capabilities in
that inner namespace, right?
Cheers,
Michael
> Eric
>
>
>> Here's an example program that makes use of the new ioctl() operations.
>>
>> 8x---8x---8x---8x---8x---8x---8x---8x---8x---8x---8x---8x---8x---8x---
>> /* ns_capable.c
>>
>> (C) 2016 Michael Kerrisk, <mtk.manpages@xxxxxxxxx>
>>
>> Licensed under the GNU General Public License v2 or later.
>>
>> Test whether a process (identified by PID) might (subject to LSM checks)
>> have capabilities in a namespace (identified by a /proc/PID/ns/xxx file).
>> */
>> #define _GNU_SOURCE
>> #include <sched.h>
>> #include <stdlib.h>
>> #include <unistd.h>
>> #include <stdio.h>
>> #include <errno.h>
>> #include <fcntl.h>
>> #include <string.h>
>> #include <sys/stat.h>
>> #include <sys/ioctl.h>
>> #include <limits.h>
>> #include <sys/capability.h>
>>
>> #ifndef NS_GET_USERNS
>> #define NSIO 0xb7
>> #define NS_GET_USERNS _IO(NSIO, 0x1)
>> #define NS_GET_PARENT _IO(NSIO, 0x2)
>> #define NS_GET_NSTYPE _IO(NSIO, 0x3)
>> #define NS_GET_OWNER_UID _IO(NSIO, 0x4)
>> #endif
>>
>> #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
>> } while (0)
>>
>> #define fatal(msg) do { fprintf(stderr, "%s\n", msg); \
>> exit(EXIT_FAILURE); } while (0)
>>
>> /* Display capabilities sets of process with specified PID */
>>
>> static void
>> show_cap(pid_t pid)
>> {
>> cap_t caps;
>> char *cap_string;
>>
>> caps = cap_get_pid(pid);
>> if (caps == NULL)
>> errExit("cap_get_proc");
>>
>> cap_string = cap_to_text(caps, NULL);
>> if (cap_string == NULL)
>> errExit("cap_to_text");
>>
>> printf("Capabilities: %s\n", cap_string);
>> }
>>
>> /* Obtain the effective UID pf the process 'pid' by
>> scanning its /proc/PID/file */
>>
>> static uid_t
>> get_euid_of_process(pid_t pid)
>> {
>> char path[PATH_MAX];
>> char line[1024];
>> int uid;
>>
>> snprintf(path, sizeof(path), "/proc/%ld/status", (long) pid);
>>
>> FILE *fp;
>> fp = fopen(path, "r");
>> if (fp == NULL)
>> errExit("fopen-/proc/PID/status");
>>
>> for (;;) {
>> if (fgets(line, sizeof(line), fp) == NULL) {
>>
>> /* Should never happen... */
>>
>> fprintf(stderr, "Failure scanning %s\n", path);
>> exit(EXIT_FAILURE);
>> }
>>
>> if (strstr(line, "Uid:") == line) {
>> sscanf(line, "Uid: %*d %d %*d %*d", &uid);
>> return uid;
>> }
>> }
>> }
>>
>> int
>> main(int argc, char *argv[])
>> {
>> int ns_fd, userns_fd, pid_userns_fd;
>> int nstype;
>> int next_fd;
>> struct stat pid_stat;
>> struct stat target_stat;
>> char *pid_str;
>> pid_t pid;
>> char path[PATH_MAX];
>>
>> if (argc < 2) {
>> fprintf(stderr, "Usage: %s PID [ns-file]\n", argv[0]);
>> fprintf(stderr, "\t'ns-file' is a /proc/PID/ns/xxxx file; "
>> "if omitted, use the namespace\n"
>> "\treferred to by standard input "
>> "(file descriptor 0)\n");
>> exit(EXIT_FAILURE);
>> }
>>
>> pid_str = argv[1];
>> pid = atoi(pid_str);
>>
>> if (argc <= 2) {
>> ns_fd = STDIN_FILENO;
>> } else {
>> ns_fd = open(argv[2], O_RDONLY);
>> if (ns_fd == -1)
>> errExit("open-ns-file");
>> }
>>
>> /* Get the relevant user namespace FD, which is 'ns_fd' if 'ns_fd' refers
>> to a user namespace, otherwise the user namespace that owns 'ns_fd' */
>>
>> nstype = ioctl(ns_fd, NS_GET_NSTYPE);
>> if (nstype == -1)
>> errExit("ioctl-NS_GET_NSTYPE");
>>
>> if (nstype == CLONE_NEWUSER) {
>> userns_fd = ns_fd;
>> } else {
>> userns_fd = ioctl(ns_fd, NS_GET_USERNS);
>> if (userns_fd == -1)
>> errExit("ioctl-NS_GET_USERNS");
>> }
>>
>> /* Obtain 'stat' info for the user namespace of the specified PID */
>>
>> snprintf(path, sizeof(path), "/proc/%s/ns/user", pid_str);
>>
>> pid_userns_fd = open(path, O_RDONLY);
>> if (pid_userns_fd == -1)
>> errExit("open-PID");
>>
>> if (fstat(pid_userns_fd, &pid_stat) == -1)
>> errExit("fstat-PID");
>>
>> /* Get 'stat' info for the target user namesapce */
>>
>> if (fstat(userns_fd, &target_stat) == -1)
>> errExit("fstat-PID");
>>
>> /* If the PID is in the target user namespace, then it has
>> whatever capabilities are in its sets. */
>>
>> if (pid_stat.st_dev == target_stat.st_dev &&
>> pid_stat.st_ino == target_stat.st_ino) {
>> printf("PID is in target namespace\n");
>> printf("Subject to LSM checks, it has the following capabilities\n");
>>
>> show_cap(pid);
>>
>> exit(EXIT_SUCCESS);
>> }
>>
>> /* Otherwise, we need to walk through the ancestors of the target
>> user namespace to see if PID is in an ancestor namespace */
>>
>> for (;;) {
>> int f;
>>
>> next_fd = ioctl(userns_fd, NS_GET_PARENT);
>>
>> if (next_fd == -1) {
>>
>> /* The error here should be EPERM... */
>>
>> if (errno != EPERM)
>> errExit("ioctl-NS_GET_PARENT");
>>
>> printf("PID is not in an ancestor namespace\n");
>> printf("It has no capabilities in the target namespace\n");
>>
>> exit(EXIT_SUCCESS);
>> }
>>
>> if (fstat(next_fd, &target_stat) == -1)
>> errExit("fstat-PID");
>>
>> /* If the 'stat' info for this user namespace matches the 'stat'
>> * info for 'next_fd', then the PID is in an ancestor namespace */
>>
>> if (pid_stat.st_dev == target_stat.st_dev &&
>> pid_stat.st_ino == target_stat.st_ino)
>> break;
>>
>> /* Next time round, get the next parent */
>>
>> f = userns_fd;
>> userns_fd = next_fd;
>> close(f);
>> }
>>
>> /* At this point, we found that PID is in an ancestor of the target
>> user namespace, and 'userns_fd' refers to the immediate descendant
>> user namespace of PID in the chain of user namespaces from PID to
>> the target user namespace. If the effective UID of PID matches the
>> owner UID of descendant user namespace, then PID has all
>> capabilities in the descendant namespace(s); otherwise, it just has
>> the capabilities that are in its sets. */
>>
>> uid_t owner_uid, uid;
>> if (ioctl(userns_fd, NS_GET_OWNER_UID, &owner_uid) == -1) {
>> perror("ioctl-NS_GET_OWNER_UID");
>> exit(EXIT_FAILURE);
>> }
>>
>> uid = get_euid_of_process(pid);
>>
>> printf("PID is in an ancestor namespace\n");
>> if (owner_uid == uid) {
>> printf("And its effective UID matches the owner "
>> "of the namespace\n");
>> printf("Subject to LSM checks, PID has all capabilities in "
>> "that namespace!\n");
>> } else {
>> printf("But its effective UID does not match the owner "
>> "of the namespace\n");
>> printf("Subject to LSM checks, it has the following capabilities\n");
>> show_cap(pid);
>> }
>>
>> exit(EXIT_SUCCESS);
>> }
>> 8x---8x---8x---8x---8x---8x---8x---8x---8x---8x---8x---8x---8x---8x---
>>
>> Michael Kerrisk (2):
>> nsfs: Add an ioctl() to return the namespace type
>> nsfs: Add an ioctl() to return owner UID of a userns
>>
>> fs/nsfs.c | 15 +++++++++++++++
>> include/uapi/linux/nsfs.h | 9 +++++++--
>> 2 files changed, 22 insertions(+), 2 deletions(-)
>>
>> --
>> 2.5.5
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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