linux-kernel - Re: ioctl CAP_LINUX_IMMUTABLE is checked in the wrong namespace

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Date:	Tue, 29 Apr 2014 14:35:34 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Marian Marinov <mm@...com>
Cc:	containers@...ts.linux-foundation.org,
	LXC development mailing-list 
	<lxc-devel@...ts.linuxcontainers.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: ioctl CAP_LINUX_IMMUTABLE is checked in the wrong namespace

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.

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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