libbb: do not die if setgid/setuid(real_id) on startup fails
Based on a patch from Steven McDonald <steven@steven-mcdonald.id.au>:
This makes 'unshare --user' work correctly in the case where the user's
shell is provided by busybox itself.
'unshare --user' creates a new user namespace without any uid mappings.
As a result, /bin/busybox is setuid nobody:nogroup within the
namespace, as that is the only user. However, since no uids are mapped,
attempting to call setgid/setuid fails, even though this would do
nothing:
$ unshare --user ./busybox.broken ash
ash: setgid: Invalid argument
'unshare --map-root-user' still works, but because Linux only allows
uid/gid mappings to be set up once, creating a root mapping makes such
a namespace useless for creating multi-user containers.
With this patch, setgid and setuid will not be called in the case where
they would do nothing, which is always the case inside a new user
namespace because all uids are effectively mapped to nobody:
$ id -u
1000
$ ls -lh busybox.fixed
-rwsr-xr-x 1 root root 826.2K May 21 00:33 busybox.fixed
$ unshare --user ./busybox.fixed ash
$ id -u
65534
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/libbb/appletlib.c b/libbb/appletlib.c
index df65849..b9fbbd1 100644
--- a/libbb/appletlib.c
+++ b/libbb/appletlib.c
@@ -681,8 +681,21 @@
if (geteuid())
bb_error_msg_and_die("must be suid to work properly");
} else if (APPLET_SUID(applet_no) == BB_SUID_DROP) {
- xsetgid(rgid); /* drop all privileges */
- xsetuid(ruid);
+ /*
+ * Drop all privileges.
+ *
+ * Don't check for errors: in normal use, they are impossible,
+ * and in special cases, exiting is harmful. Example:
+ * 'unshare --user' when user's shell is also from busybox.
+ *
+ * 'unshare --user' creates a new user namespace without any
+ * uid mappings. Thus, busybox binary is setuid nobody:nogroup
+ * within the namespace, as that is the only user. However,
+ * since no uids are mapped, calls to setgid/setuid
+ * fail (even though they would do nothing).
+ */
+ setgid(rgid);
+ setuid(ruid);
}
# if ENABLE_FEATURE_SUID_CONFIG
ret: ;