Re: [PATCH] fs: add an ioctl to get an owning userns for a superblock

[Andrei Vagin]

On Tue, May 09, 2017 at 07:34:00PM -0500, Eric W. Biederman wrote:
> Andrei Vagin <avagin@xxxxxxxxxx> writes:
> 
> > The introduced ioctl returns a file descriptor that refers to a owning
> > user namespace for a superblock which is associated with a target file
> > descriptor.
> >
> > EPERM is returned if the current process doesn't have CAP_SYS_ADMIN in
> > the returned user namespace.
> >
> > This information is required to dump and restore mount namespaces. We
> > need to know to which user namespace a superblock is belonged to.
> >
> > We already have the SIOCGSKNS ioctl for sockets to get a network
> > namespace, so it looks reasonable to use the same interface for
> > superblocks too.
> >
> > This functionality can be useful for users in order to understand
> > a running system.
> 
> This will probably work.  And the capability check eases any concerns
> I might have that this would be a trivial information leak.
> 
> That said can we hold off just a little bit.  If open_fs work actually
> turns into a real interface that would seem to be the perfect place
> to stick this functionality.

Sure, we can. Do you know any place where to read more information about
open_fs? I think I have heared a few times about this idea, but it would be
good to get more details.

Thanks,
Andrei

> 
> Eric
> 
> >
> > Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> > Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> > Signed-off-by: Andrei Vagin <avagin@xxxxxxxxxx>
> > ---
> >  fs/ioctl.c              | 23 +++++++++++++++++++++++
> >  include/uapi/linux/fs.h |  2 ++
> >  2 files changed, 25 insertions(+)
> >
> > diff --git a/fs/ioctl.c b/fs/ioctl.c
> > index 569db68..22bbf37 100644
> > --- a/fs/ioctl.c
> > +++ b/fs/ioctl.c
> > @@ -16,6 +16,8 @@
> >  #include <linux/buffer_head.h>
> >  #include <linux/falloc.h>
> >  #include <linux/sched/signal.h>
> > +#include <linux/proc_fs.h>
> > +#include <linux/user_namespace.h>
> >  
> >  #include "internal.h"
> >  
> > @@ -614,6 +616,25 @@ static int ioctl_file_dedupe_range(struct file *file, void __user *arg)
> >  	return ret;
> >  }
> >  
> > +static struct ns_common *get_sb_userns(struct ns_common *ns_common)
> > +{
> > +	struct user_namespace *ns;
> > +
> > +	ns = container_of(ns_common, struct user_namespace, ns);
> > +
> > +	return &get_user_ns(ns)->ns;
> > +}
> > +
> > +static int ioctl_fs_sb_userns(struct file *filp)
> > +{
> > +	struct super_block *sb = file_inode(filp)->i_sb;
> > +
> > +	if (!ns_capable(sb->s_user_ns, CAP_SYS_ADMIN))
> > +		return -EPERM;
> > +
> > +	return open_related_ns(&sb->s_user_ns->ns, get_sb_userns);
> > +}
> > +
> >  /*
> >   * When you add any new common ioctls to the switches above and below
> >   * please update compat_sys_ioctl() too.
> > @@ -677,6 +698,8 @@ int do_vfs_ioctl(struct file *filp, unsigned int fd, unsigned int cmd,
> >  
> >  	case FIDEDUPERANGE:
> >  		return ioctl_file_dedupe_range(filp, argp);
> > +	case FS_IOC_SB_USERNS:
> > +		return ioctl_fs_sb_userns(filp);
> >  
> >  	default:
> >  		if (S_ISREG(inode->i_mode))
> > diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
> > index 33423aa..26ef2d5 100644
> > --- a/include/uapi/linux/fs.h
> > +++ b/include/uapi/linux/fs.h
> > @@ -246,6 +246,8 @@ struct fsxattr {
> >  #define FICLONE		_IOW(0x94, 9, int)
> >  #define FICLONERANGE	_IOW(0x94, 13, struct file_clone_range)
> >  #define FIDEDUPERANGE	_IOWR(0x94, 54, struct file_dedupe_range)
> > +/* Get a file descriptor to an owning userns for a superblock */
> > +#define FS_IOC_SB_USERNS		_IOR('X', 55, int)
> >  
> >  #define	FS_IOC_GETFLAGS			_IOR('f', 1, long)
> >  #define	FS_IOC_SETFLAGS			_IOW('f', 2, long)