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lxc-attach: User namespaces: Use init's user & group id when attaching

When attaching to a container with a user namespace, try to detect the
user and group ids of init via /proc and attach as that same user. Only
if that is unsuccessful, fall back to (0, 0).

Signed-off-by: Christian Seiler <christian@iwakd.de>
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1 parent 21da991 commit cb3e61fa37431373e52602e6b26a3e2ade9e2941 @chris-se chris-se committed with Serge Hallyn Mar 6, 2013
Showing with 60 additions and 4 deletions.
  1. +47 −0 src/lxc/attach.c
  2. +2 −0 src/lxc/attach.h
  3. +11 −4 src/lxc/lxc_attach.c
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@@ -428,3 +428,50 @@ char *lxc_attach_getpwshell(uid_t uid)
exit(-1);
}
}
+
+void lxc_attach_get_init_uidgid(uid_t* init_uid, gid_t* init_gid)
+{
+ FILE *proc_file;
+ char proc_fn[MAXPATHLEN];
+ char *line = NULL;
+ size_t line_bufsz = 0;
+ int ret;
+ long value = -1;
+ uid_t uid = (uid_t)-1;
+ gid_t gid = (gid_t)-1;
+
+ /* read capabilities */
+ snprintf(proc_fn, MAXPATHLEN, "/proc/%d/status", 1);
+
+ proc_file = fopen(proc_fn, "r");
+ if (!proc_file)
+ return;
+
+ while (getline(&line, &line_bufsz, proc_file) != -1) {
+ /* format is: real, effective, saved set user, fs
+ * we only care about real uid
+ */
+ ret = sscanf(line, "Uid: %ld", &value);
+ if (ret != EOF && ret > 0) {
+ uid = (uid_t) value;
+ } else {
+ ret = sscanf(line, "Gid: %ld", &value);
+ if (ret != EOF && ret > 0)
+ gid = (gid_t) value;
+ }
+ if (uid != (uid_t)-1 && gid != (gid_t)-1)
+ break;
+ }
+
+ fclose(proc_file);
+ free(line);
+
+ /* only override arguments if we found something */
+ if (uid != (uid_t)-1)
+ *init_uid = uid;
+ if (gid != (gid_t)-1)
+ *init_gid = gid;
+
+ /* TODO: we should also parse supplementary groups and use
+ * setgroups() to set them */
+}
View
@@ -40,4 +40,6 @@ extern int lxc_attach_drop_privs(struct lxc_proc_context_info *ctx);
extern char *lxc_attach_getpwshell(uid_t uid);
+extern void lxc_attach_get_init_uidgid(uid_t* init_uid, gid_t* init_gid);
+
#endif
View
@@ -418,13 +418,20 @@ int main(int argc, char *argv[])
lxc_sync_fini(handler);
if (namespace_flags & CLONE_NEWUSER) {
- /* XXX FIXME this should get the uid of the container init and setuid to that */
- /* XXX FIXME or perhaps try to map in the lxc-attach caller's uid? */
- if (setgid(0)) {
+ uid_t init_uid = 0;
+ gid_t init_gid = 0;
+
+ /* ignore errors, we will fall back to root in that case
+ * (/proc was not mounted etc.)
+ */
+ lxc_attach_get_init_uidgid(&init_uid, &init_gid);
+
+ /* try to set the uid/gid combination */
+ if (setgid(init_gid)) {
SYSERROR("switching to container gid");
return -1;
}
- if (setuid(0)) {
+ if (setuid(init_uid)) {
SYSERROR("switching to container uid");
return -1;
}

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