Re: ioctl CAP_LINUX_IMMUTABLE is checked in the wrong namespace

[Serge Hallyn <serge.hallyn@ubuntu.com>]

Quoting Theodore Ts'o (tytso@mit.edu):
> 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?

> No?  Otherwise, pretty much every single use of capable() would be
> broken, not just this once instances in ext4/ioctl.c.
> 
> 					- Ted
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