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From: ebiederm@xmission.com (Eric W. Biederman)
To: "Theodore Y. Ts'o" <tytso@mit.edu>
Cc: 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: Thu, 24 May 2018 18:23:30 -0500	[thread overview]
Message-ID: <87y3g8y6x9.fsf@xmission.com> (raw)
In-Reply-To: <20180524214617.GG7712@thunk.org> (Theodore Y. Ts'o's message of "Thu, 24 May 2018 17:46:17 -0400")

"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.

Eric

  reply	other threads:[~2018-05-24 23:23 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 [this message]
2018-05-25  3:57     ` Dave Chinner
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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