Rehashing My File Permissions Understanding(or lack of it)

[Paul Howarth]

Mark Sargent wrote:
> Paul Howarth wrote:
> 
>> Jay Paulson wrote:
>>
>>> I was under the impression that changing the umask was a possible 
>>> security risk.  Am I correct in thinking that?
>>
>>
>>
>> Possibly, possibly not. Using a umask of 002 instead of 022 is 
>> something that Red Hat/Fedora specifically cater for. What this means 
>> is that woth a umask of 002, files are created with group write 
>> permissions by default, so if your default group is shared with a 
>> number of other people then they will be able to write to your files 
>> by default. However, in Red Hat/Fedora, every new user is created with 
>> their own group by default, which isn't shared with any other user. So 
>> enabling group write permission isn't a big issue. What this then lets 
>> you do is to create a separate group for shared data, and then 
>> everyone's default umask being 002 (if set that way) then makes it 
>> easy for all members to create and edit files with this shared groupid.
>>
>> Paul.
>>
> Hi All,
> 
> so, in theory, if there were a way to set a umask specifically for a 
> certain group, it'd be great. For example; when user xman, who is a 
> member of say, share2 group, creates a new file in a particular dir, the 
> new file would be writable by all within that same group. Would that 
> just make things too messy, OR, am I just not getting it.? Cheers.

You're right that this would be great, but unfortunately I don't know of 
  any way of implementing it.

Well, actually that's not quite true. Using samba you can do things like 
this by forcing permissions and uids/gids. But for access to local files 
and directories, I don't know of a way of doing this in a 
directory-specific way.

Do note thought that if your users all have their own groups, as is the 
default in Red Hat/Fedora, you should be safe to set the umask to 002 
for all users. If you then create a directory /path/to/dir and do:

# chgrp share2 /path/to/dir
# chmod g+s /path/to/dir

then any files/directories created in that directory should get the 
right group ID and permissions.

Paul.