Re: [RFC 17/20] ima: Use integrity_admin_ns_capable() to check corresponding capability

From: Stefan Berger
Date: Thu Dec 02 2021 - 12:55:09 EST


On 12/2/21 02:16, Denis Semakin wrote:
Obviously the main goal by adding new capability was to avoid the using CAP_SYS_ADMIN (IOW superuser)
to manage IMA stuff, that was also about security granularity. It's good if CAP_MAC_ADMIN will be enough for doing IMA related things (write policies and extended attributes).
But for me it's a little bit unclear how to deal with unprivileged users: assuming there's no CAP_INTEGRITY_ADMIN but CAP_MAC_ADMIN was set up, so in this case user can control any LSM (seLinux, SMACK, etc) and IMA (policies, xattrs). What if .. for some systems there would be some requirements that will allow to touch LSM but do not change any IMA (integrity) things? A user can set up any IMA policy (it's about the system integrity), modify IMA related xattrs but it's forbidden to change seLinux policies and e.g. SMACK labels... May be it's unreal scenario of course... but I guess it's not 100% impossible.

If we can introduce a new capability I would use CAP_INTEGRITY_ADMIN, if not CAP_MAC_ADMIN.


   Stefan


Best regards,
Denis


-----Original Message-----
From: James Bottomley [mailto:jejb@xxxxxxxxxxxxx]
Sent: Wednesday, December 1, 2021 10:29 PM
To: Stefan Berger <stefanb@xxxxxxxxxxxxx>; linux-integrity@xxxxxxxxxxxxxxx
Cc: zohar@xxxxxxxxxxxxx; serge@xxxxxxxxxx; christian.brauner@xxxxxxxxxx; containers@xxxxxxxxxxxxxxx; dmitry.kasatkin@xxxxxxxxx; ebiederm@xxxxxxxxxxxx; Krzysztof Struczynski <krzysztof.struczynski@xxxxxxxxxx>; Roberto Sassu <roberto.sassu@xxxxxxxxxx>; mpeters@xxxxxxxxxx; lhinds@xxxxxxxxxx; lsturman@xxxxxxxxxx; puiterwi@xxxxxxxxxx; jamjoom@xxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; paul@xxxxxxxxxxxxxx; rgb@xxxxxxxxxx; linux-security-module@xxxxxxxxxxxxxxx; jmorris@xxxxxxxxx; Denis Semakin <denis.semakin@xxxxxxxxxx>
Subject: Re: [RFC 17/20] ima: Use integrity_admin_ns_capable() to check corresponding capability

On Wed, 2021-12-01 at 12:35 -0500, Stefan Berger wrote:
On 12/1/21 11:58, James Bottomley wrote:
On Tue, 2021-11-30 at 11:06 -0500, Stefan Berger wrote:
From: Denis Semakin <denis.semakin@xxxxxxxxxx>

Use integrity_admin_ns_capable() to check corresponding capability
to allow read/write IMA policy without CAP_SYS_ADMIN but with
CAP_INTEGRITY_ADMIN.

Signed-off-by: Denis Semakin <denis.semakin@xxxxxxxxxx>
---
security/integrity/ima/ima_fs.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/security/integrity/ima/ima_fs.c
b/security/integrity/ima/ima_fs.c index fd2798f2d224..6766bb8262f2
100644
--- a/security/integrity/ima/ima_fs.c
+++ b/security/integrity/ima/ima_fs.c
@@ -393,7 +393,7 @@ static int ima_open_policy(struct inode
*inode, struct file *filp)
#else
if ((filp->f_flags & O_ACCMODE) != O_RDONLY)
return -EACCES;
- if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN))
+ if (!integrity_admin_ns_capable(ns->user_ns))
so this one is basically replacing what you did in RFC 16/20, which
seems a little redundant.

The question I'd like to ask is: is there still a reason for needing
CAP_INTEGRITY_ADMIN? My thinking is that now IMA is pretty much
tied to requiring a user (and a mount, because of
securityfs_ns) namespace, there might not be a pressing need for an
admin capability separated from CAP_SYS_ADMIN because the owner of
the user namespace passes the ns_capable(..., CAP_SYS_ADMIN) check.
The rationale in
Casey suggested using CAP_MAC_ADMIN, which I think would also work.

CAP_MAC_ADMIN (since Linux 2.6.25)
Allow MAC configuration or state changes. Implemented
for
the Smack Linux Security Module (LSM).


Down the road I think we should cover setting file extended attributes
with the same capability as well for when a user signs files or
installs packages with file signatures. A container runtime could
hold CAP_SYS_ADMIN while setting up a container and mounting
filesystems and drop it for the first process started there. Since we
are using the user namespace to spawn an IMA namespace, we would then
require CAP_SYSTEM_ADMIN to be left available so that the user can do
IMA related stuff in the container (set or append to the policy, write
file signatures). I am not sure whether that should be the case or
rather give the user something finer grained, such as CAP_MAC_ADMIN.
So, it's about granularity...
It's possible ... any orchestration system that doesn't enter a user
namespace has to strictly regulate capabilities. I'm probably biased
because I always use a user_ns so I never really had to mess with capabilities.

https://kernsec.org/wiki/index.php/IMA_Namespacing_design_considerat
ions

Is effectively "because CAP_SYS_ADMIN is too powerful" but that's no
longer true of the user namespace owner. It only passes the
ns_capable() check not the capable() one, so while it does get
CAP_SYS_ADMIN, it can only use it in a few situations which
represent quite a power reduction already.
At least docker containers drop CAP_SYS_ADMIN.
Well docker doesn't use the user_ns. But even given that, CAP_SYS_ADMIN is always dropped for most container systems. What happens when you enter a user namespace is the ns_capable( ...,
CAP_SYS_ADMIN) check returns true if you're the owner of the user_ns, in the same way it would for root. So effectively entering a user namespace without CAP_SYS_ADMIN but mapping the owner id to 0 (what unshare -r --user does) gives you back a form of CAP_SYS_ADMIN that responds only in the places in the kernel that have a ns_capable() check instead of a capable() one (most of the places you list below).
This is the principle of how unprivileged containers actually work ...
and the source of some of our security problems if you get back an ability to do something you shouldn't be allowed to do as an unprivileged user.

I am not sure what the decision was based on but probably they don't
want to give the user what is not absolutely necessary, but usage of
user namespaces (with IMA namespaces) would kind of force it to be
available then to do IMA-related stuff ...

Following this man page here
https://man7.org/linux/man-pages/man7/user_namespaces.7.html

CAP_SYS_ADMIN in a user namespace is about

- bind-mounting filesystems

- mounting /proc filesystems

- creating nested user namespaces

- configuring UTS namespace

- configuring whether setgroups() can be used

- usage of setns()


Do we want to add '- only way of *setting up* IMA related stuff' to
this list?
I don't see why not, but other container people should weigh in because, as I said, I mostly use the user namespace and unprivileged containers and don't bother with capabilities.

James