Question

How might it be possible to rig a system such that some removable media, like a USB stick, can be mounted by a user without any need for root privileges, and yet be made private for that user? I’m thinking of some multiuser situation, like a school or library, where there are many terminals. I sit down at a terminal, plug in my USB stick, mount it, without needing any privileges, have full access to it, and yet it is completely private to me. I know I can gain privacy my mounting the stick in my home directory, but I’d need root privileges to make the mount. In fstab I can use the ‘user,noauto’ option, which lets me, or anyone else, mount, OTOH, then, once mounted, the sick is visible to anyone. Is there some way of making the device itself private? chmod’ing the contents is not sufficient since other users can still do things like write new files. I think this should be doable but I’m damned if I can figure it out.

Answer 1

This issue does not lie in the removable media, but rather in the filesystem. The FAT filesystem does not support individual access control.

You can solve your problem by defining default access rights in your fstab. From the manpage:

   umask=value
          Set the umask (the bitmask of  the  permissions  that  are  not
          present). The default is the umask of the current process.  The
          value is given in octal.

   dmask=value
          Set the umask applied to directories only.  The default is  the
          umask of the current process.  The value is given in octal.

   fmask=value
          Set  the  umask  applied to regular files only.  The default is
          the umask of the current process.  The value is given in octal.

So, e.g. “user,noauto,umask=077” should take all rights away from anybody else then the user performing the mount. Note that this is already the default dmask on most systems! Maybe something is wrong with your setup.