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libbb: do not die if setgid/setuid(real_id) on startup fails

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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>
This commit is contained in:
Denys Vlasenko 2017-07-10 09:17:43 +02:00
parent 9de9c871bf
commit 12a4f9afe7
1 changed files with 15 additions and 2 deletions
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17
libbb/appletlib.c
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@ -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: ;