Re: [RFC][PATCH] unshare: Fix --map-root-user to work on new kernels

[Karel Zak <kzak@redhat.com>]

On Fri, Dec 19, 2014 at 06:28:45AM -0600, Eric W. Biederman wrote:
> >> This may also have some affect on the setgroups(0, NULL) case of
> >> nsenter as well.
... 
> The best would be to call setgroups(0, NULL) before entering the user
> namespace (so root can always clear their groups), and call setgroups(0,
> NULL) after entering the user namespace (as currently happens).  If both
> setgroups(0, NULL) calls fail then complain.
> 
> nsenter as currently constructed can not enter a user namespaces that
> does not map uid 0 and gid 0.   So not handling setgroups=deny for
> non-root users in seems reasonable.
> 
> What looks compelling to me is a --preserve-credentials option to
> nsenter that would not touch uids or gids.  A --preserve-credentials
> option will allow nsenter to enter all manner of user namespaces
> irrespective of they are configured.

 Implemented. Please, review.

    Karel


>From 6e38747286e958e5c0f6060e803ea916bb6be756 Mon Sep 17 00:00:00 2001
From: Karel Zak <kzak@redhat.com>
Date: Thu, 8 Jan 2015 12:52:43 +0100
Subject: [PATCH] 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.

CC: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Karel Zak <kzak@redhat.com>
---
 sys-utils/nsenter.1 |  4 ++++
 sys-utils/nsenter.c | 27 +++++++++++++++++++++------
 2 files changed, 25 insertions(+), 6 deletions(-)

diff --git a/sys-utils/nsenter.1 b/sys-utils/nsenter.1
index 487f731..0b1fceb 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 50f77f3..b029f80 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"));
-- 
1.9.3


-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com