A confined sftp user

[Erinn Looney-Triggs]

My company asked me today to set up a user that is allowed only to
upload files via sftp. This got me thinking, an sftp user has shell
access as well, of course, and this can lead to all kinds of interesting
things (the kernel privilege escalation from last week comes to mind).

I figured it might be appropriate to run this user as a confined user,
at least at a minimum running the user as user_u would block a lot of
options, or perhaps a different user I haven't researched them all yet.

Now the question is, would SELinux be an appropriate place for an sftp_u
user? What I am envisioning is a confined user, that allows only the
sftp subsystem to be run and files to be uploaded to the confined users
homedir. It seems to me that SELinux would be a good fit for this, but I
am merely an amateur here :).

Anyone ever done anything like this? Would this be an easy thing?

There are of course other options, folks have written programs to
confine a user to only uploading via sftp, rssh and others.

-Erinn

[Dominick Grift]

On Tue, 2012-02-07 at 16:31 -0900, Erinn Looney-Triggs wrote:
> My company asked me today to set up a user that is allowed only to
> upload files via sftp. This got me thinking, an sftp user has shell
> access as well, of course, and this can lead to all kinds of interesting
> things (the kernel privilege escalation from last week comes to mind).
>
> I figured it might be appropriate to run this user as a confined user,
> at least at a minimum running the user as user_u would block a lot of
> options, or perhaps a different user I haven't researched them all yet.

I don't think these users need a shell. You could probably use notty
option in their authorized_keys file.

Try guest_u (useradd -Z guest_u joe)

guest_t is pretty much made for this purpose. Though its not perfect but
in combination with other security measures it pretty good.

Think firewalling (because last time i checked these users are able to
do udp flood attacks to the outside), ip checking, pki auth, resource
management like cgroups etc etc
> Now the question is, would SELinux be an appropriate place for an sftp_u
> user? What I am envisioning is a confined user, that allows only the
> sftp subsystem to be run and files to be uploaded to the confined users
> homedir. It seems to me that SELinux would be a good fit for this, but I
> am merely an amateur here :).
>
> Anyone ever done anything like this? Would this be an easy thing?

Not easy but not that hard either. Basically one could clone the source
policy for the least privileged login user available and modify that to
your requirements.

Whether it is worth the trouble that depends on your requirements.
> There are of course other options, folks have written programs to
> confine a user to only uploading via sftp, rssh and others.
>
> -Erinn
>
> --
> selinux mailing list
> [e-mail]
> https://admin.fedoraproject.org/mailman/listinfo/selinux

[Miroslav Grepl]

On 02/08/2012 01:31 AM, Erinn Looney-Triggs wrote:
> My company asked me today to set up a user that is allowed only to
> upload files via sftp. This got me thinking, an sftp user has shell
> access as well, of course, and this can lead to all kinds of interesting
> things (the kernel privilege escalation from last week comes to mind).
>
> I figured it might be appropriate to run this user as a confined user,
> at least at a minimum running the user as user_u would block a lot of
> options, or perhaps a different user I haven't researched them all yet.
>
> Now the question is, would SELinux be an appropriate place for an sftp_u
> user? What I am envisioning is a confined user, that allows only the
> sftp subsystem to be run and files to be uploaded to the confined users
> homedir. It seems to me that SELinux would be a good fit for this, but I
> am merely an amateur here :).
>
> Anyone ever done anything like this? Would this be an easy thing?
>
> There are of course other options, folks have written programs to
> confine a user to only uploading via sftp, rssh and others.
>
> -Erinn
>
> --
> selinux mailing list
> [e-mail]
> https://admin.fedoraproject.org/mailman/listinfo/selinux

What OS?

We have sftp+chroot+SELinux in Fedora16/17/RHEL6.2. You could chroot 
users in their home directories and then after sftp on a machine, a user 
will run in the "chroot_user_t" domain.

This domain has these accesses by default

userdom_read_user_home_content_files(chroot_user_t)
userdom_read_inherited_user_home_content_files(chroot_user_t)
userdom_read_user_home_content_symlinks(chroot_user_t)
userdom_exec_user_home_content_files(chroot_user_t

and the "ssh_chroot_rw_homedirs" boolean.

[Dominick Grift]

On Wed, 2012-02-08 at 14:15 +0000, Miroslav Grepl wrote:
> What OS?
>
> We have sftp+chroot+SELinux in Fedora16/17/RHEL6.2. You could chroot
> users in their home directories and then after sftp on a machine, a
> user will run in the "chroot_user_t" domain.
>
> This domain has these accesses by default
>
> userdom_read_user_home_content_files(chroot_user_t)
> userdom_read_inherited_user_home_content_files(chroot_user_t)
> userdom_read_user_home_content_symlinks(chroot_user_t)
> userdom_exec_user_home_content_files(chroot_user_t
>
> and the "ssh_chroot_rw_homedirs" boolean.

You might want to write a blog about how this is supposed to work and
how chroot_user_t differs from sftpd_t.
> --
> selinux mailing list
> [e-mail]
> https://admin.fedoraproject.org/mailman/listinfo/selinux

[Miroslav Grepl]

On 02/08/2012 03:10 PM, Dominick Grift wrote:
> On Wed, 2012-02-08 at 14:15 +0000, Miroslav Grepl wrote:
> > What OS?
> >
> > We have sftp+chroot+SELinux in Fedora16/17/RHEL6.2. You could chroot
> > users in their home directories and then after sftp on a machine, a
> > user will run in the "chroot_user_t" domain.
> >
> > This domain has these accesses by default
> >
> > userdom_read_user_home_content_files(chroot_user_t)
> > userdom_read_inherited_user_home_content_files(chroot_user_t)
> > userdom_read_user_home_content_symlinks(chroot_user_t)
> > userdom_exec_user_home_content_files(chroot_user_t
> >
> > and the "ssh_chroot_rw_homedirs" boolean.
>
> You might want to write a blog about how this is supposed to work and
> how chroot_user_t differs from sftpd_t.

Yes, you read my mind. I have it on my TODO list. Basically, there is no 
longer sftpd_t. There is just chroot_user_t for "Chroot" option and 
userdomain context for internal-sftp subsystem without chroot.
> > --
> > selinux mailing list
> > [e-mail]
> > https://admin.fedoraproject.org/mailman/listinfo/selinux
>
> --
> selinux mailing list
> [e-mail]
> https://admin.fedoraproject.org/mailman/listinfo/selinux

[Erinn Looney-Triggs]

On 02/08/2012 05:15 AM, Miroslav Grepl wrote:
> On 02/08/2012 01:31 AM, Erinn Looney-Triggs wrote:
> > My company asked me today to set up a user that is allowed only to
> > upload files via sftp. This got me thinking, an sftp user has shell
> > access as well, of course, and this can lead to all kinds of interesting
> > things (the kernel privilege escalation from last week comes to mind).
> >
> > I figured it might be appropriate to run this user as a confined user,
> > at least at a minimum running the user as user_u would block a lot of
> > options, or perhaps a different user I haven't researched them all yet.
> >
> > Now the question is, would SELinux be an appropriate place for an sftp_u
> > user? What I am envisioning is a confined user, that allows only the
> > sftp subsystem to be run and files to be uploaded to the confined users
> > homedir. It seems to me that SELinux would be a good fit for this, but I
> > am merely an amateur here :).
> >
> > Anyone ever done anything like this? Would this be an easy thing?
> >
> > There are of course other options, folks have written programs to
> > confine a user to only uploading via sftp, rssh and others.
> >
> > -Erinn
> >
> > --
> > selinux mailing list
> > [e-mail] 
> > https://admin.fedoraproject.org/mailman/listinfo/selinux
>
> What OS?
>
> We have sftp+chroot+SELinux in Fedora16/17/RHEL6.2. You could chroot
> users in their home directories and then after sftp on a machine, a user
> will run in the "chroot_user_t" domain.
>
> This domain has these accesses by default
>
> userdom_read_user_home_content_files(chroot_user_t)
> userdom_read_inherited_user_home_content_files(chroot_user_t)
> userdom_read_user_home_content_symlinks(chroot_user_t)
> userdom_exec_user_home_content_files(chroot_user_t
>
> and the "ssh_chroot_rw_homedirs" boolean.

RHEL 6.2, it looks like between your suggestions and Dominick's
suggestions I can probably put together a pretty good little sandbox for
an sftp user, without of course, having to become the master of the
universe that can write policy ;).

Thanks for all the good info,

-Erinn

[Miroslav Grepl]

On 02/08/2012 06:38 PM, Erinn Looney-Triggs wrote:
> On 02/08/2012 05:15 AM, Miroslav Grepl wrote:
> > On 02/08/2012 01:31 AM, Erinn Looney-Triggs wrote:
> > > My company asked me today to set up a user that is allowed only to
> > > upload files via sftp. This got me thinking, an sftp user has shell
> > > access as well, of course, and this can lead to all kinds of interesting
> > > things (the kernel privilege escalation from last week comes to mind).
> > >
> > > I figured it might be appropriate to run this user as a confined user,
> > > at least at a minimum running the user as user_u would block a lot of
> > > options, or perhaps a different user I haven't researched them all yet.
> > >
> > > Now the question is, would SELinux be an appropriate place for an sftp_u
> > > user? What I am envisioning is a confined user, that allows only the
> > > sftp subsystem to be run and files to be uploaded to the confined users
> > > homedir. It seems to me that SELinux would be a good fit for this, but I
> > > am merely an amateur here :).
> > >
> > > Anyone ever done anything like this? Would this be an easy thing?
> > >
> > > There are of course other options, folks have written programs to
> > > confine a user to only uploading via sftp, rssh and others.
> > >
> > > -Erinn
> > >
> > > --
> > > selinux mailing list
> > > [e-mail]
> > > https://admin.fedoraproject.org/mailman/listinfo/selinux
> >
> > What OS?
> >
> > We have sftp+chroot+SELinux in Fedora16/17/RHEL6.2. You could chroot
> > users in their home directories and then after sftp on a machine, a user
> > will run in the "chroot_user_t" domain.
> >
> > This domain has these accesses by default
> >
> > userdom_read_user_home_content_files(chroot_user_t)
> > userdom_read_inherited_user_home_content_files(chroot_user_t)
> > userdom_read_user_home_content_symlinks(chroot_user_t)
> > userdom_exec_user_home_content_files(chroot_user_t
> >
> > and the "ssh_chroot_rw_homedirs" boolean.
>
> RHEL 6.2, it looks like between your suggestions and Dominick's
> suggestions I can probably put together a pretty good little sandbox for
> an sftp user, without of course, having to become the master of the
> universe that can write policy ;).
>
> Thanks for all the good info,
>
> -Erinn

Petr Lautrbach (openssh package maintainer) is just writing a blog how 
to setup it. I am going to post his blog tomorrow.

[Erinn Looney-Triggs]

On 02/08/2012 08:58 AM, Miroslav Grepl wrote:
> On 02/08/2012 06:38 PM, Erinn Looney-Triggs wrote:
> > On 02/08/2012 05:15 AM, Miroslav Grepl wrote:
> > > On 02/08/2012 01:31 AM, Erinn Looney-Triggs wrote:
> > > > My company asked me today to set up a user that is allowed only to
> > > > upload files via sftp. This got me thinking, an sftp user has shell
> > > > access as well, of course, and this can lead to all kinds of
> > > > interesting
> > > > things (the kernel privilege escalation from last week comes to mind).
> > > >
> > > > I figured it might be appropriate to run this user as a confined user,
> > > > at least at a minimum running the user as user_u would block a lot of
> > > > options, or perhaps a different user I haven't researched them all yet.
> > > >
> > > > Now the question is, would SELinux be an appropriate place for an
> > > > sftp_u
> > > > user? What I am envisioning is a confined user, that allows only the
> > > > sftp subsystem to be run and files to be uploaded to the confined users
> > > > homedir. It seems to me that SELinux would be a good fit for this,
> > > > but I
> > > > am merely an amateur here :).
> > > >
> > > > Anyone ever done anything like this? Would this be an easy thing?
> > > >
> > > > There are of course other options, folks have written programs to
> > > > confine a user to only uploading via sftp, rssh and others.
> > > >
> > > > -Erinn
> > > >
> > > > -- 
> > > > selinux mailing list
> > > > [e-mail]
> > > > https://admin.fedoraproject.org/mailman/listinfo/selinux
> > >
> > > What OS?
> > >
> > > We have sftp+chroot+SELinux in Fedora16/17/RHEL6.2. You could chroot
> > > users in their home directories and then after sftp on a machine, a user
> > > will run in the "chroot_user_t" domain.
> > >
> > > This domain has these accesses by default
> > >
> > > userdom_read_user_home_content_files(chroot_user_t)
> > > userdom_read_inherited_user_home_content_files(chroot_user_t)
> > > userdom_read_user_home_content_symlinks(chroot_user_t)
> > > userdom_exec_user_home_content_files(chroot_user_t
> > >
> > > and the "ssh_chroot_rw_homedirs" boolean.
> >
> > RHEL 6.2, it looks like between your suggestions and Dominick's
> > suggestions I can probably put together a pretty good little sandbox for
> > an sftp user, without of course, having to become the master of the
> > universe that can write policy ;).
> >
> > Thanks for all the good info,
> >
> > -Erinn
>
> Petr Lautrbach (openssh package maintainer) is just writing a blog how
> to setup it. I am going to post his blog tomorrow.

Well that is just wonderful, thanks Miroslav and thank Petr for me.

-Erinn

[Miroslav Grepl]

On 02/08/2012 10:13 PM, Erinn Looney-Triggs wrote:
> On 02/08/2012 08:58 AM, Miroslav Grepl wrote:
> > On 02/08/2012 06:38 PM, Erinn Looney-Triggs wrote:
> > > On 02/08/2012 05:15 AM, Miroslav Grepl wrote:
> > > > On 02/08/2012 01:31 AM, Erinn Looney-Triggs wrote:
> > > > > My company asked me today to set up a user that is allowed only to
> > > > > upload files via sftp. This got me thinking, an sftp user has shell
> > > > > access as well, of course, and this can lead to all kinds of
> > > > > interesting
> > > > > things (the kernel privilege escalation from last week comes to mind).
> > > > >
> > > > > I figured it might be appropriate to run this user as a confined user,
> > > > > at least at a minimum running the user as user_u would block a lot of
> > > > > options, or perhaps a different user I haven't researched them all yet.
> > > > >
> > > > > Now the question is, would SELinux be an appropriate place for an
> > > > > sftp_u
> > > > > user? What I am envisioning is a confined user, that allows only the
> > > > > sftp subsystem to be run and files to be uploaded to the confined users
> > > > > homedir. It seems to me that SELinux would be a good fit for this,
> > > > > but I
> > > > > am merely an amateur here :).
> > > > >
> > > > > Anyone ever done anything like this? Would this be an easy thing?
> > > > >
> > > > > There are of course other options, folks have written programs to
> > > > > confine a user to only uploading via sftp, rssh and others.
> > > > >
> > > > > -Erinn
> > > > >
> > > > > -- 
> > > > > selinux mailing list
> > > > > [e-mail]
> > > > > https://admin.fedoraproject.org/mailman/listinfo/selinux
> > > >
> > > > What OS?
> > > >
> > > > We have sftp+chroot+SELinux in Fedora16/17/RHEL6.2. You could chroot
> > > > users in their home directories and then after sftp on a machine, a user
> > > > will run in the "chroot_user_t" domain.
> > > >
> > > > This domain has these accesses by default
> > > >
> > > > userdom_read_user_home_content_files(chroot_user_t)
> > > > userdom_read_inherited_user_home_content_files(chroot_user_t)
> > > > userdom_read_user_home_content_symlinks(chroot_user_t)
> > > > userdom_exec_user_home_content_files(chroot_user_t
> > > >
> > > > and the "ssh_chroot_rw_homedirs" boolean.
> > >
> > > RHEL 6.2, it looks like between your suggestions and Dominick's
> > > suggestions I can probably put together a pretty good little sandbox for
> > > an sftp user, without of course, having to become the master of the
> > > universe that can write policy ;).
> > >
> > > Thanks for all the good info,
> > >
> > > -Erinn
> >
> > Petr Lautrbach (openssh package maintainer) is just writing a blog how
> > to setup it. I am going to post his blog tomorrow.
>
> Well that is just wonderful, thanks Miroslav and thank Petr for me.

Here is:

http://bachradsusi.livejournal.com/2239.html
> -Erinn

[Dominick Grift]

On Thu, 2012-02-09 at 16:30 +0100, Miroslav Grepl wrote:
> Here is:
>
> http://bachradsusi.livejournal.com/2239.html
> > -Erinn

Thanks verified.

Only thing to keep in mind is to restorecon -R -v /home/sftponlyuser

made a screencast of the process while verifying:
http://youtu.be/KJVlis1uwIg

[Miroslav Grepl]

On 02/09/2012 05:53 PM, Dominick Grift wrote:
> On Thu, 2012-02-09 at 16:30 +0100, Miroslav Grepl wrote:
> > Here is:
> >
> > http://bachradsusi.livejournal.com/2239.html
> > > -Erinn
>
> Thanks verified.
>
> Only thing to keep in mind is to restorecon -R -v /home/sftponlyuser

Ah, I think this is copy/paste issue. Will tell Petr to fix it.

Thanks.
> made a screencast of the process while verifying:
> http://youtu.be/KJVlis1uwIg
>
> --
> selinux mailing list
> [e-mail]
> https://admin.fedoraproject.org/mailman/listinfo/selinux