Documenting the requirement of CAP_SETFCAP to map UID 0

[Michael Kerrisk (man-pages)]

Hi Serge

On 8/11/21 1:58 AM, Serge E. Hallyn wrote:
> On Sun, Aug 08, 2021 at 11:09:30AM +0200, Michael Kerrisk (man-pages) wrote:
>> Hello Serge,
>>
Hello Serge,


>> Your commit:
>>
>> [[
>> commit db2e718a47984b9d71ed890eb2ea36ecf150de18
>> Author: Serge E. Hallyn <serge at hallyn.com>
>> Date:   Tue Apr 20 08:43:34 2021 -0500
>>
>>     capabilities: require CAP_SETFCAP to map uid 0
>> ]]
>>
>> added a new requirement when updating a UID map a user namespace
>> with a value of '0 0 *'.
>>
>> Kir sent a patch to briefly document this change, but I think much more
>> should be written. I've attempted to do so. Could you tell me whether the
>> following text (to be added in user_namespaces(7)) is accurate please:
> 
> Sorry for the delay - this did not go into my main mailbox.
> 
> The text looks good.  Thanks!

Thanks for checking it!

Cheers,

Michael

>> [[
>>       In  order  for  a  process  to  write  to  the /proc/[pid]/uid_map
>>        (/proc/[pid]/gid_map) file, all of the following requirements must
>>        be met:
>>
>>        [...]
>>
>>        4. If  updating  /proc/[pid]/uid_map to create a mapping that maps
>>           UID 0 in the parent namespace, then one of the  following  must
>>           be true:
>>
>>           *  if  writing process is in the parent user namespace, then it
>>              must have the CAP_SETFCAP capability in that user namespace;
>>              or
>>
>>           *  if  the writing process is in the child user namespace, then
>>              the process that created the user namespace  must  have  had
>>              the CAP_SETFCAP capability when the namespace was created.
>>
>>           This rule has been in place since Linux 5.12.  It eliminates an
>>           earlier security bug whereby a UID 0  process  that  lacks  the
>>           CAP_SETFCAP capability, which is needed to create a binary with
>>           namespaced file capabilities (as described in capabilities(7)),
>>           could  nevertheless  create  such  a  binary,  by the following
>>           steps:
>>
>>           *  Create a new user namespace with the identity mapping (i.e.,
>>              UID  0 in the new user namespace maps to UID 0 in the parent
>>              namespace), so that UID 0 in both namespaces  is  equivalent
>>              to the same root user ID.
>>
>>           *  Since  the  child process has the CAP_SETFCAP capability, it
>>              could create a binary with namespaced file capabilities that
>>              would  then  be  effective in the parent user namespace (be‐
>>              cause the root user IDs are the same in the two namespaces).
>>
>>        [...]
>> ]]
>>
>> Thanks,
>>
>> Michael
>>
>> -- 
>> Michael Kerrisk
>> Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
>> Linux/UNIX System Programming Training: http://man7.org/training/


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/