[lxc-devel] [PATCH 1/1] cgroup: prevent DOS when a hierachy is mounted multiple times

[Serge Hallyn]

When starting a container, we walk through all cgroup mounts looking
for a unique directory name we can use for this container.  If the
name we are trying is in use, we try another name.  If it is not in
use in the first mount we check, we need to check other hierarchies
as it may exist there.  But we weren't checking whether we have already
checked a subsystem - so that if freezer was mounted twice, we would
create it in the first mount, see it exists in the second, so start
over trying in the second mount.

To fix this, keep track of which subsystems we have already checked,
and do not re-check.

(See http://pad.lv/1176287 for a bug report)

Note we still need to add, at the next: label, the removal of the
directories we've already created.  I'm keeping that for later as
it's far lower priority than this fix, and I don't want to risk
introducing a regression for that.

Signed-off-by: Serge Hallyn <serge.hallyn at ubuntu.com>
---
 src/lxc/cgroup.c | 122 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 122 insertions(+)

diff --git a/src/lxc/cgroup.c b/src/lxc/cgroup.c
index 0c93703..a6b5d75 100644
--- a/src/lxc/cgroup.c
+++ b/src/lxc/cgroup.c
@@ -504,6 +504,103 @@ static void set_clone_children(const char *mntdir)
 	fclose(fout);
 }
 
+static char *get_all_cgroups(void)
+{
+	FILE *f;
+	char *line = NULL, *ret = NULL;
+	size_t len;
+	int first = 1;
+
+	/* read the list of subsystems from the kernel */
+	f = fopen("/proc/cgroups", "r");
+	if (!f)
+		return NULL;
+
+	while (getline(&line, &len, f) != -1) {
+		char *c;
+		int oldlen, newlen, inc;
+
+		/* skip the first line */
+		if (first) {
+			first=0;
+			continue;
+		}
+
+		c = strchr(line, '\t');
+		if (!c)
+			continue;
+		*c = '\0';
+
+		oldlen = ret ? strlen(ret) : 0;
+		newlen = oldlen + strlen(line) + 2;
+		ret = realloc(ret, newlen);
+		if (!ret)
+			goto out;
+		inc = snprintf(ret + oldlen, newlen, ",%s", line);
+		if (inc < 0 || inc >= newlen) {
+			free(ret);
+			ret = NULL;
+			goto out;
+		}
+	}
+
+out:
+	fclose(f);
+	return ret;
+}
+
+static int in_cgroup_list(char *s, char *list)
+{
+	char *token, *str, *saveptr;
+
+	if (!list || !s)
+		return 0;
+
+	for (str = strdupa(list); (token = strtok_r(str, ",", &saveptr)); str = NULL) {
+		if (strcmp(s, token) == 0)
+			return 1;
+	}
+
+	return 0;
+}
+
+static int have_visited(char *opts, char *visited, char *allcgroups)
+{
+	char *str, *s, *token;
+
+	for (str = strdupa(opts); (token = strtok_r(str, ",", &s)); str = NULL) {
+		if (!in_cgroup_list(token, allcgroups))
+			continue;
+		if (visited && in_cgroup_list(token, visited))
+			return 1;
+	}
+
+	return 0;
+}
+
+static int record_visited(char *opts, char **visitedp, char *allcgroups)
+{
+	char *s, *token, *str;
+	int oldlen, newlen, ret;
+
+	for (str = strdupa(opts); (token = strtok_r(str, ",", &s)); str = NULL) {
+		if (!in_cgroup_list(token, allcgroups))
+			continue;
+		if (*visitedp && in_cgroup_list(token, *visitedp))
+			continue;
+		oldlen = (*visitedp) ? strlen(*visitedp) : 0;
+		newlen = oldlen + strlen(token) + 2;
+		(*visitedp) = realloc(*visitedp, newlen);
+		if (!(*visitedp))
+			return -1;
+		ret = snprintf((*visitedp)+oldlen, newlen, ",%s", token);
+		if (ret < 0 || ret >= newlen)
+			return -1;
+	}
+
+	return 0;
+}
+
 /*
  * Make sure the 'cgroup group' exists, so that we don't have to worry about
  * that later.
@@ -592,16 +689,29 @@ char *lxc_cgroup_path_create(const char *lxcgroup, const char *name)
 	char tail[12];
 	FILE *file = NULL;
 	struct mntent mntent_r;
+	char *allcgroups = get_all_cgroups();
+	char *visited = NULL;
 
 	char buf[LARGE_MAXPATHLEN] = {0};
 
 	if (create_lxcgroups(lxcgroup) < 0)
 		return NULL;
 
+	if (!allcgroups)
+		return NULL;
+
 again:
+        if (visited) {
+                /* we're checking for a new name, so start over with all cgroup
+                 * mounts */
+                free(visited);
+                visited = NULL;
+        }
 	file = setmntent(MTAB, "r");
 	if (!file) {
 		SYSERROR("failed to open %s", MTAB);
+                if (allcgroups)
+                        free(allcgroups);
 		return NULL;
 	}
 
@@ -617,6 +727,12 @@ again:
 		if (!mount_has_subsystem(&mntent_r))
 			continue;
 
+		/* make sure we haven't checked this subsystem already */
+		if (have_visited(mntent_r.mnt_opts, visited, allcgroups))
+			continue;
+		if (record_visited(mntent_r.mnt_opts, &visited, allcgroups) < 0)
+			goto fail;
+
 		/* find unused mnt_dir + lxcgroup + name + -$i */
 		ret = snprintf(path, MAXPATHLEN, "%s/%s/%s%s", mntent_r.mnt_dir,
 			       lxcgroup ? lxcgroup : "lxc", name, tail);
@@ -641,6 +757,9 @@ again:
 		goto fail;
 
 	retpath = strdup(path);
+	free(allcgroups);
+	if (visited)
+		free(visited);
 
 	return retpath;
 
@@ -651,6 +770,9 @@ next:
 
 fail:
 	endmntent(file);
+	free(allcgroups);
+	if (visited)
+		free(visited);
 	return NULL;
 }
 
-- 
1.8.1.2