[ebiederm at xmission.com: [REVIEW][PATCH 3/3] vfs: Fix a regression in mounting proc]

[Andy Whitcroft]

On Fri, Nov 29, 2013 at 04:07:52PM -0600, Serge Hallyn wrote:
> Hi,
> 
> This is the second patch needed in trusty's kernel for containers.  It
> is in
> https://git.kernel.org/cgit/linux/kernel/git/ebiederm/user-namespace.git/log/?h=for-next
> (and I assume linux-next).  This fixes a regression recently introduced
> which prevents using user namespaces for containers.
> 
> ----- Forwarded message from "Eric W. Biederman" <ebiederm at xmission.com> -----
> 
> Date: Tue, 26 Nov 2013 16:17:36 -0800
> From: "Eric W. Biederman" <ebiederm at xmission.com>
> To: "Serge E. Hallyn" <serge at hallyn.com>
> Cc: Gao feng <gaofeng at cn.fujitsu.com>, Containers <containers at lists.linux-foundation.org>,
> 	linux-fsdevel at vger.kernel.org, Aditya Kali <adityakali at google.com>, Oleg Nesterov
> 	<oleg at redhat.com>, Andy Lutomirski <luto at amacapital.net>
> Subject: [REVIEW][PATCH 3/3] vfs: Fix a regression in mounting proc
> 
> 
> Gao feng <gaofeng at cn.fujitsu.com> reported that commit
> e51db73532955dc5eaba4235e62b74b460709d5b
> userns: Better restrictions on when proc and sysfs can be mounted
> caused a regression on mounting a new instance of proc in a mount
> namespace created with user namespace privileges, when binfmt_misc
> is mounted on /proc/sys/fs/binfmt_misc.
> 
> This is an unintended regression caused by the absolutely bogus empty
> directory check in fs_fully_visible.  The check fs_fully_visible replaced
> didn't even bother to attempt to verify proc was fully visible and
> hiding proc files with any kind of mount is rare.  So for now fix
> the userspace regression by allowing directory with nlink == 1
> as /proc/sys/fs/binfmt_misc has.
> 
> I will have a better patch but it is not stable material, or
> last minute kernel material.  So it will have to wait.
> 
> Cc: stable at vger.kernel.org
> Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>
> ---
>  fs/namespace.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/namespace.c b/fs/namespace.c
> index ac2ce8a766e1..be32ebccdeb1 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -2886,7 +2886,7 @@ bool fs_fully_visible(struct file_system_type *type)
>  			struct inode *inode = child->mnt_mountpoint->d_inode;
>  			if (!S_ISDIR(inode->i_mode))
>  				goto next;
> -			if (inode->i_nlink != 2)
> +			if (inode->i_nlink > 2)
>  				goto next;
>  		}
>  		visible = true;

Looks reasonable.  Applied to trusty unstable.

-apw