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

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Date:	Wed, 30 Apr 2014 00:16:41 +0000
From:	Serge Hallyn <serge.hallyn@...ntu.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Marian Marinov <mm@...com>, containers@...ts.linux-foundation.org,
	Ted Ts'o <tytso@....edu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	lxc-devel <lxc-devel@...ts.sourceforge.net>
Subject: Re: ioctl CAP_LINUX_IMMUTABLE is checked in the wrong namespace

Quoting Andy Lutomirski (luto@...capital.net):
> On 04/29/2014 03:29 PM, Serge Hallyn wrote:
> > Quoting Marian Marinov (mm-108MBtLGafw@...lic.gmane.org):
> >> On 04/30/2014 01:02 AM, Serge Hallyn wrote:
> >>> Quoting Marian Marinov (mm-108MBtLGafw@...lic.gmane.org):
> >>>> On 04/29/2014 09:52 PM, Serge Hallyn wrote:
> >>>>> Quoting Theodore Ts'o (tytso-3s7WtUTddSA@...lic.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.
> >>
> >> If you are ok with this patch, I will fix all file systems and send patches.
> > 
> > Sounds good, thanks.
> > 
> >> Signed-off-by: Marian Marinov <mm-NV7Lj0SOnH0@...lic.gmane.org>
> > 
> > Acked-by: Serge E. Hallyn <serge.hallyn-GeWIH/nMZzLQT0dZR+AlfA@...lic.gmane.org>
> 
> Wait, what?
> 
> Inodes aren't owned by user namespaces; they're owned by users.  And any
> user can arrange to have a user namespace in which they pass an
> inode_capable check on any inode that they own.
> 
> Presumably there's a reason that CAP_SYS_IMMUTABLE is needed.  If this

Sigh, yeah...  I just dont' understand what it is.  But you're right.

> gets merged, then it would be better to just drop CAP_SYS_IMMUTABLE
> entirely.
> 
> Nacked-by: Andy Lutomirski <luto@...capital.net>

I forget the details, but there was another case where I wanted to
have the userns which 'owns' the whole fs available.  I guess we'd
have to check against that instead of using inode_capable.

-serge
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