From 12a4f9afe702b89c616610d3a29bf08257c6bace Mon Sep 17 00:00:00 2001 From: Denys Vlasenko Date: Mon, 10 Jul 2017 09:17:43 +0200 Subject: libbb: do not die if setgid/setuid(real_id) on startup fails Based on a patch from Steven McDonald : 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 --- libbb/appletlib.c | 17 +++++++++++++++-- 1 file changed, 15 insertions(+), 2 deletions(-) (limited to 'libbb/appletlib.c') diff --git a/libbb/appletlib.c b/libbb/appletlib.c index df6584978..b9fbbd1f2 100644 --- a/libbb/appletlib.c +++ b/libbb/appletlib.c @@ -681,8 +681,21 @@ static void check_suid(int applet_no) 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: ; -- cgit v1.2.3