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From: Dave Chinner <david@fromorbit.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: "Theodore Y. Ts'o" <tytso@mit.edu>,
	Linux Containers <containers@lists.linux-foundation.org>,
	linux-fsdevel@vger.kernel.org,
	Seth Forshee <seth.forshee@canonical.com>,
	"Serge E. Hallyn" <serge@hallyn.com>,
	Christian Brauner <christian@brauner.io>,
	linux-kernel@vger.kernel.org
Subject: Re: [REVIEW][PATCH 0/6] Wrapping up the vfs support for unprivileged mounts
Date: Fri, 25 May 2018 13:57:16 +1000	[thread overview]
Message-ID: <20180525035716.GE10363@dastard> (raw)
In-Reply-To: <87y3g8y6x9.fsf@xmission.com>

On Thu, May 24, 2018 at 06:23:30PM -0500, Eric W. Biederman wrote:
> "Theodore Y. Ts'o" <tytso@mit.edu> writes:
> 
> > On Wed, May 23, 2018 at 06:22:56PM -0500, Eric W. Biederman wrote:
> >> 
> >> Very slowly the work has been progressing to ensure the vfs has the
> >> necessary support for mounting filesystems without privilege.
> >
> > What's the thinking behind how system administrators and/or file
> > systems would configure whether or not a particular file system type
> > will be allowed to be mounted w/o privilege?
> 
> The mechanism is .fs_flags in file_system_type.   If the FS_USERNS_MOUNT
> flag is set then root in a user namespace (AKA an unprivileged user)
> will be allowed to mount to mount the filesystem.
> 
> There are very real concerns about attacking a filesystem with an
> invalid filesystem image, or by a malicious protocol speaker.  So I
> don't want to enable anything without the file system maintainers
> consent and without a reasonable expecation that neither a system wide
> denial of service attack nor a privilege escalation attack is possible
> from if the filesystem is enabled.
> 
> So at a practical level what we have in the vfs is the non-fuse specific
> bits that enable unprivileged mounts of fuse.  Things like handling
> of unmapped uid and gids, how normally trusted xattrs are dealt with,
> etc.
> 
> A big practical one for me is that if either the uid or gid is not
> mapped the vfs avoids writing to the inode.
> 
> Right now my practical goal is to be able to say: "Go run your
> filesystem in userspace with fuse if you want stronger security
> guarantees."  I think that will be enough to make removable media
> reasonably safe from privilege escalation attacks.
> 
> There is enough code in most filesystems that I don't know what our
> chances of locking down very many of them are.  But I figure a few more
> of them are possible.

I'm not sure we need to - fusefs-lkl gives users the ability to
mount any of the kernel filesystems via fuse without us needing to
support unprivileged kernel mounts for those filesystems.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

  reply	other threads:[~2018-05-25  3:57 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-23 23:22 Eric W. Biederman
2018-05-23 23:25 ` [REVIEW][PATCH 1/6] vfs: Don't allow changing the link count of an inode with an invalid uid or gid Eric W. Biederman
2018-05-24 12:58   ` Seth Forshee
2018-05-24 22:30     ` Christian Brauner
2018-05-23 23:25 ` [REVIEW][PATCH 2/6] vfs: Allow userns root to call mknod on owned filesystems Eric W. Biederman
2018-05-24 13:55   ` Seth Forshee
2018-05-24 16:55     ` Eric W. Biederman
2018-05-24 17:22       ` Seth Forshee
2018-05-24 19:12   ` Christian Brauner
2018-05-23 23:25 ` [REVIEW][PATCH 3/6] fs: Allow superblock owner to replace invalid owners of inodes Eric W. Biederman
2018-05-23 23:41   ` [REVIEW][PATCH v2 " Eric W. Biederman
2018-05-24 22:30     ` Christian Brauner
2018-05-23 23:25 ` [REVIEW][PATCH 4/6] fs: Allow superblock owner to access do_remount_sb() Eric W. Biederman
2018-05-24 15:58   ` Christian Brauner
2018-05-24 16:45     ` Eric W. Biederman
2018-05-24 17:28       ` Christian Brauner
2018-05-23 23:25 ` [REVIEW][PATCH 5/6] capabilities: Allow privileged user in s_user_ns to set security.* xattrs Eric W. Biederman
2018-05-24 15:57   ` Christian Brauner
2018-05-23 23:25 ` [REVIEW][PATCH 6/6] fs: Allow CAP_SYS_ADMIN in s_user_ns to freeze and thaw filesystems Eric W. Biederman
2018-05-24 15:59   ` Christian Brauner
2018-05-24 21:46 ` [REVIEW][PATCH 0/6] Wrapping up the vfs support for unprivileged mounts Theodore Y. Ts'o
2018-05-24 23:23   ` Eric W. Biederman
2018-05-25  3:57     ` Dave Chinner [this message]
2018-05-25  4:06       ` Darrick J. Wong
2018-05-29 13:12       ` Eric W. Biederman
2018-05-29 22:17         ` Dave Chinner
2018-05-30  2:34           ` Eric W. Biederman
2018-05-30  4:34             ` Dave Chinner
2018-05-29 15:40 ` Dongsu Park

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