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

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Message-ID: <20140429222913.GD28410@ubuntumail>
Date:	Tue, 29 Apr 2014 22:29:13 +0000
From:	Serge Hallyn <serge.hallyn@...ntu.com>
To:	Marian Marinov <mm@...com>
Cc:	Theodore Ts'o <tytso@....edu>,
	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

Quoting Marian Marinov (mm@...com):
> On 04/30/2014 01:02 AM, Serge Hallyn wrote:
> >Quoting Marian Marinov (mm@...com):
> >>On 04/29/2014 09:52 PM, Serge Hallyn wrote:
> >>>Quoting Theodore Ts'o (tytso@....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?
> >>>
> >>
> >>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@...u.biz>

Acked-by: Serge E. Hallyn <serge.hallyn@...ntu.com>

> ---
>  fs/ext4/ioctl.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
> index d011b69..9418634 100644
> --- a/fs/ext4/ioctl.c
> +++ b/fs/ext4/ioctl.c
> @@ -265,7 +265,7 @@ long ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
>                  * This test looks nicer. Thanks to Pauline Middelink
>                  */
>                 if ((flags ^ oldflags) & (EXT4_APPEND_FL | EXT4_IMMUTABLE_FL)) {
> -                   if (!capable(CAP_LINUX_IMMUTABLE))
> +                 if (!inode_capable(inode, CAP_LINUX_IMMUTABLE))
>                                 goto flags_out;
>                 }
> 
> ---
> 1.8.4
> 
> Marian
> 
> 
> >
> >thanks,
> >-serge
> >
> 
> 
> -- 
> Marian Marinov
> Founder & CEO of 1H Ltd.
> Jabber/GTalk: hackman@...ber.org
> ICQ: 7556201
> Mobile: +359 886 660 270
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