nsenter: add --preserve-credentials and cleanup setgroups() usage

The new option --preserve-credentials completely disables all
operations related to UIGs and GIDs.

The patch also calls setgroups() before we enter user namespace (so
root can always clear their groups) and after we enter user namespace
(to detect /proc/self/setgroups "deny"). If both fail then nsenter
complains.

Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Karel Zak <kzak@redhat.com>

2 files changed, 25 insertions, 6 deletions

diff --git a/sys-utils/nsenter.1 b/sys-utils/nsenter.1
index 487f731dc..0b1fceb36 100644
--- a/sys-utils/nsenter.1
+++ b/sys-utils/nsenter.1
@@ -136,6 +136,10 @@ Set the user ID which will be used in the entered namespace.
 .BR nsenter (1)
 always sets UID for user namespaces, the default is 0.
 .TP
+\fB\-\-preserve-credentials\fR
+Don't modify UID and GID when enter user namespace. The default is to
+drops supplementary groups and sets GID and UID to 0.
+.TP
 \fB\-r\fR, \fB\-\-root\fR[=\fIdirectory\fR]
 Set the root directory.  If no directory is specified, set the root directory to
 the root directory of the target process.  If directory is specified, set the
diff --git a/sys-utils/nsenter.c b/sys-utils/nsenter.c
index 50f77f381..b029f8087 100644
--- a/sys-utils/nsenter.c
+++ b/sys-utils/nsenter.c
@@ -78,6 +78,7 @@ static void usage(int status)
 	fputs(_(" -U, --user[=<file>]    enter user namespace\n"), out);
 	fputs(_(" -S, --setuid <uid>     set uid in entered namespace\n"), out);
 	fputs(_(" -G, --setgid <gid>     set gid in entered namespace\n"), out);
+	fputs(_("     --preserve-credentials do not touch uids or gids\n"), out);
 	fputs(_(" -r, --root[=<dir>]     set the root directory\n"), out);
 	fputs(_(" -w, --wd[=<dir>]       set the working directory\n"), out);
 	fputs(_(" -F, --no-fork          do not fork before exec'ing <program>\n"), out);
@@ -165,6 +166,9 @@ static void continue_as_child(void)
 
 int main(int argc, char *argv[])
 {
+	enum {
+		OPT_PRESERVE_CRED = CHAR_MAX + 1
+	};
 	static const struct option longopts[] = {
 		{ "help", no_argument, NULL, 'h' },
 		{ "version", no_argument, NULL, 'V'},
@@ -180,11 +184,12 @@ int main(int argc, char *argv[])
 		{ "root", optional_argument, NULL, 'r' },
 		{ "wd", optional_argument, NULL, 'w' },
 		{ "no-fork", no_argument, NULL, 'F' },
+		{ "preserve-credentials", no_argument, NULL, OPT_PRESERVE_CRED },
 		{ NULL, 0, NULL, 0 }
 	};
 
 	struct namespace_file *nsfile;
-	int c, namespaces = 0;
+	int c, namespaces = 0, setgroups_nerrs = 0, preserve_cred = 0;
 	bool do_rd = false, do_wd = false, force_uid = false, force_gid = false;
 	int do_fork = -1; /* unknown yet */
 	uid_t uid = 0;
@@ -267,6 +272,9 @@ int main(int argc, char *argv[])
 			else
 				do_wd = true;
 			break;
+		case OPT_PRESERVE_CRED:
+			preserve_cred = 1;
+			break;
 		default:
 			usage(EXIT_FAILURE);
 		}
@@ -292,6 +300,17 @@ int main(int argc, char *argv[])
 		namespaces |= nsfile->nstype;
 	}
 
+	/* for user namespaces we always set UID and GID (default is 0)
+	 * and clear root's groups if --preserve-credentials is no specified */
+	if ((namespaces & CLONE_NEWUSER) && !preserve_cred) {
+		force_uid = true, force_gid = true;
+
+		/* We call setgroups() before and after we enter user namespace,
+		 * let's complain only if both fail */
+		if (setgroups(0, NULL) != 0)
+			setgroups_nerrs++;
+	}
+
 	/*
 	 * Now that we know which namespaces we want to enter, enter them.
 	 */
@@ -342,12 +361,8 @@ int main(int argc, char *argv[])
 	if (do_fork == 1)
 		continue_as_child();
 
-	/* for user namespaces we always set UID and GID (default is 0) */
-	if (namespaces & CLONE_NEWUSER)
-		force_uid = true, force_gid = true;
-
 	if (force_uid || force_gid) {
-		if (force_gid && setgroups(0, NULL))		/* drop supplementary groups */
+		if (force_gid && setgroups(0, NULL) != 0 && setgroups_nerrs)	/* drop supplementary groups */
 			err(EXIT_FAILURE, _("setgroups failed"));
 		if (force_gid && setgid(gid) < 0)		/* change GID */
 			err(EXIT_FAILURE, _("setgid failed"));