capabilities: require CAP_SETFCAP to map uid 0 (v3.3)

cap_setfcap is required to create file capabilities.

Since 8db6c34f1dbc ("Introduce v3 namespaced file capabilities"), a
process running as uid 0 but without cap_setfcap is able to work around
this as follows: unshare a new user namespace which maps parent uid 0
into the child namespace.  While this task will not have new
capabilities against the parent namespace, there is a loophole due to
the way namespaced file capabilities are represented as xattrs.  File
capabilities valid in userns 1 are distinguished from file capabilities
valid in userns 2 by the kuid which underlies uid 0.  Therefore the
restricted root process can unshare a new self-mapping namespace, add a
namespaced file capability onto a file, then use that file capability in
the parent namespace.

To prevent that, do not allow mapping parent uid 0 if the process which
opened the uid_map file does not have CAP_SETFCAP, which is the capability
for setting file capabilities.

As a further wrinkle:  a task can unshare its user namespace, then
open its uid_map file itself, and map (only) its own uid.  In this
case we do not have the credential from before unshare,  which was
potentially more restricted.  So, when creating a user namespace, we
record whether the creator had CAP_SETFCAP.  Then we can use that
during map_write().

With this patch:

1. Unprivileged user can still unshare -Ur

ubuntu@caps:~$ unshare -Ur
root@caps:~# logout

2. Root user can still unshare -Ur

ubuntu@caps:~$ sudo bash
root@caps:/home/ubuntu# unshare -Ur
root@caps:/home/ubuntu# logout

3. Root user without CAP_SETFCAP cannot unshare -Ur:

root@caps:/home/ubuntu# /sbin/capsh --drop=cap_setfcap --
root@caps:/home/ubuntu# /sbin/setcap cap_setfcap=p /sbin/setcap
unable to set CAP_SETFCAP effective capability: Operation not permitted
root@caps:/home/ubuntu# unshare -Ur
unshare: write failed /proc/self/uid_map: Operation not permitted

Note: an alternative solution would be to allow uid 0 mappings by
processes without CAP_SETFCAP, but to prevent such a namespace from
writing any file capabilities.  This approach can be seen here:


Commit 95ebabde382 ("capabilities: Don't allow writing ambiguous v3 file
capabilities") tried to fix the issue by preventing v3 fscaps to be
written to disk when the root uid would map to the same uid in nested
user namespaces. This led to regressions for various workloads. For
example, see [1]. Ultimately this is a valid use-case we have to support
meaning we had to revert this change in 3b0c2d3eaa83 ("Revert
95ebabde382c ("capabilities: Don't allow writing ambiguous v3 file


Signed-off-by: Serge Hallyn <>
Reviewed-by: Andrew G. Morgan <>
Tested-by: Christian Brauner <>
Reviewed-by: Christian Brauner <>
Tested-by: Giuseppe Scrivano <>
Cc: "Eric W. Biederman" <>

   * fix logic in the case of writing to another task's uid_map
   * rename 'ns' to 'map_ns', and make a file_ns local variable
   * use /* comments */
   * update the CAP_SETFCAP comment in capability.h
   * rename parent_unpriv to parent_can_setfcap (and reverse the
   * remove printks
   * clarify (i hope) the code comments
   * update capability.h comment
   * renamed parent_can_setfcap to parent_could_setfcap
   * made the check its own disallowed_0_mapping() fn
   * moved the check into new_idmap_permitted
   * rename disallowed_0_mapping to verify_root_mapping
   * change verify_root_mapping to Christian's suggested flow
   * correct+clarify comments: parent uid 0 mapping to any
     child uid is a problem.
   * remove unused lower_first variable.
3 files changed