newgidmap: enforce setgroups=deny if self-mapping a group

This is necessary to match the kernel-side policy of "self-mapping in a
user namespace is fine, but you cannot drop groups" -- a policy that was
created in order to stop user namespaces from allowing trivial privilege
escalation by dropping supplementary groups that were "blacklisted" from
certain paths.

This is the simplest fix for the underlying issue, and effectively makes
it so that unless a user has a valid mapping set in /etc/subgid (which
only administrators can modify) -- and they are currently trying to use
that mapping -- then /proc/$pid/setgroups will be set to deny. This
workaround is only partial, because ideally it should be possible to set
an "allow_setgroups" or "deny_setgroups" flag in /etc/subgid to allow
administrators to further restrict newgidmap(1).

We also don't write anything in the "allow" case because "allow" is the
default, and users may have already written "deny" even if they
technically are allowed to use setgroups. And we don't write anything if
the setgroups policy is already "deny".

Ref: https://bugs.launchpad.net/ubuntu/+source/shadow/+bug/1729357
Fixes: CVE-2018-7169
Reported-by: Craig Furman <craig.furman89@gmail.com>
Signed-off-by: Aleksa Sarai <asarai@suse.de>

src/newgidmap.c

@@ -46,32 +46,37 @@
  */
 const char *Prog;
 
-static bool verify_range(struct passwd *pw, struct map_range *range)
+
+static bool verify_range(struct passwd *pw, struct map_range *range, bool *allow_setgroups)
 {
 	/* An empty range is invalid */
 	if (range->count == 0)
 		return false;
 
-	/* Test /etc/subgid */
+	/* Test /etc/subgid. If the mapping is valid then we allow setgroups. */
-	if (have_sub_gids(pw->pw_name, range->lower, range->count))
+	if (have_sub_gids(pw->pw_name, range->lower, range->count)) {
+		*allow_setgroups = true;
 		return true;
+	}
 
-	/* Allow a process to map its own gid */
+	/* Allow a process to map its own gid. */
-	if ((range->count == 1) && (pw->pw_gid == range->lower))
+	if ((range->count == 1) && (pw->pw_gid == range->lower)) {
+		/* noop -- if setgroups is enabled already we won't disable it. */
 		return true;
+	}
 
 	return false;
 }
 
 static void verify_ranges(struct passwd *pw, int ranges,
-	struct map_range *mappings)
+	struct map_range *mappings, bool *allow_setgroups)
 {
 	struct map_range *mapping;
 	int idx;
 
 	mapping = mappings;
 	for (idx = 0; idx < ranges; idx++, mapping++) {
-		if (!verify_range(pw, mapping)) {
+		if (!verify_range(pw, mapping, allow_setgroups)) {
 			fprintf(stderr, _( "%s: gid range [%lu-%lu) -> [%lu-%lu) not allowed\n"),
 				Prog,
 				mapping->upper,
@@ -89,6 +94,70 @@ static void usage(void)
 	exit(EXIT_FAILURE);
 }
 
+void write_setgroups(int proc_dir_fd, bool allow_setgroups)
+{
+	int setgroups_fd;
+	char *policy, policy_buffer[4096];
+
+	/*
+	 * Default is "deny", and any "allow" will out-rank a "deny". We don't
+	 * forcefully write an "allow" here because the process we are writing
+	 * mappings for may have already set themselves to "deny" (and "allow"
+	 * is the default anyway). So allow_setgroups == true is a noop.
+	 */
+	policy = "deny\n";
+	if (allow_setgroups)
+		return;
+
+	setgroups_fd = openat(proc_dir_fd, "setgroups", O_RDWR|O_CLOEXEC);
+	if (setgroups_fd < 0) {
+		/*
+		 * If it's an ENOENT then we are on too old a kernel for the setgroups
+		 * code to exist. Emit a warning and bail on this.
+		 */
+		if (ENOENT == errno) {
+			fprintf(stderr, _("%s: kernel doesn't support setgroups restrictions\n"), Prog);
+			goto out;
+		}
+		fprintf(stderr, _("%s: couldn't open process setgroups: %s\n"),
+			Prog,
+			strerror(errno));
+		exit(EXIT_FAILURE);
+	}
+
+	/*
+	 * Check whether the policy is already what we want. /proc/self/setgroups
+	 * is write-once, so attempting to write after it's already written to will
+	 * fail.
+	 */
+	if (read(setgroups_fd, policy_buffer, sizeof(policy_buffer)) < 0) {
+		fprintf(stderr, _("%s: failed to read setgroups: %s\n"),
+			Prog,
+			strerror(errno));
+		exit(EXIT_FAILURE);
+	}
+	if (!strncmp(policy_buffer, policy, strlen(policy)))
+		goto out;
+
+	/* Write the policy. */
+	if (lseek(setgroups_fd, 0, SEEK_SET) < 0) {
+		fprintf(stderr, _("%s: failed to seek setgroups: %s\n"),
+			Prog,
+			strerror(errno));
+		exit(EXIT_FAILURE);
+	}
+	if (dprintf(setgroups_fd, "%s", policy) < 0) {
+		fprintf(stderr, _("%s: failed to setgroups %s policy: %s\n"),
+			Prog,
+			policy,
+			strerror(errno));
+		exit(EXIT_FAILURE);
+	}
+
+out:
+	close(setgroups_fd);
+}
+
 /*
  * newgidmap - Set the gid_map for the specified process
  */
@@ -103,6 +172,7 @@ int main(int argc, char **argv)
 	struct stat st;
 	struct passwd *pw;
 	int written;
+	bool allow_setgroups = false;
 
 	Prog = Basename (argv[0]);
 
@@ -145,7 +215,7 @@ int main(int argc, char **argv)
 				(unsigned long) getuid ()));
 		return EXIT_FAILURE;
 	}
-	
+
 	/* Get the effective uid and effective gid of the target process */
 	if (fstat(proc_dir_fd, &st) < 0) {
 		fprintf(stderr, _("%s: Could not stat directory for target %u\n"),
@@ -177,8 +247,9 @@ int main(int argc, char **argv)
 	if (!mappings)
 		usage();
 
-	verify_ranges(pw, ranges, mappings);
+	verify_ranges(pw, ranges, mappings, &allow_setgroups);
 
+	write_setgroups(proc_dir_fd, allow_setgroups);
 	write_mapping(proc_dir_fd, ranges, mappings, "gid_map");
 	sub_gid_close();