Re: ioctl CAP_LINUX_IMMUTABLE is checked in the wrong namespace

[Serge Hallyn <serge.hallyn-GeWIH/nMZzLQT0dZR+AlfA@public.gmane.org>]

Quoting Marian Marinov (mm-108MBtLGafw@public.gmane.org):
> On 04/29/2014 09:52 PM, Serge Hallyn wrote:
> >Quoting Theodore Ts'o (tytso-3s7WtUTddSA@public.gmane.org):
> >>On Tue, Apr 29, 2014 at 04:49:14PM +0300, Marian Marinov wrote:
> >>>
> >>>I'm proposing a fix to this, by replacing the capable(CAP_LINUX_IMMUTABLE)
> >>>check with ns_capable(current_cred()->user_ns, CAP_LINUX_IMMUTABLE).
> >>
> >>Um, wouldn't it be better to simply fix the capable() function?
> >>
> >>/**
> >>  * capable - Determine if the current task has a superior capability in effect
> >>  * @cap: The capability to be tested for
> >>  *
> >>  * Return true if the current task has the given superior capability currently
> >>  * available for use, false if not.
> >>  *
> >>  * This sets PF_SUPERPRIV on the task if the capability is available on the
> >>  * assumption that it's about to be used.
> >>  */
> >>bool capable(int cap)
> >>{
> >>	return ns_capable(&init_user_ns, cap);
> >>}
> >>EXPORT_SYMBOL(capable);
> >>
> >>The documentation states that it is for "the current task", and I
> >>can't imagine any use case, where user namespaces are in effect, where
> >>using init_user_ns would ever make sense.
> >
> >the init_user_ns represents the user_ns owning the object, not the
> >subject.
> >
> >The patch by Marian is wrong.  Anyone can do 'clone(CLONE_NEWUSER)',
> >setuid(0), execve, and end up satisfying 'ns_capable(current_cred()->userns,
> >CAP_SYS_IMMUTABLE)' by definition.
> >
> >So NACK to that particular patch.  I'm not sure, but IIUC it should be
> >safe to check against the userns owning the inode?
> >
> 
> So what you are proposing is to replace 'ns_capable(current_cred()->userns, CAP_SYS_IMMUTABLE)' with
> 'inode_capable(inode, CAP_SYS_IMMUTABLE)' ?
> 
> I agree that this is more sane.

Right, and I think the two operations you're looking at seem sane
to allow.

thanks,
-serge