Hi Casey,
On 6/21/22 7:19 PM, Casey Schaufler wrote:
On 6/21/2022 4:39 PM, Frederick Lawler wrote:
While creating a LSM BPF MAC policy to block user namespace creation, we
used the LSM cred_prepare hook because that is the closest hook to
prevent
a call to create_user_ns().
The calls look something like this:
cred = prepare_creds()
security_prepare_creds()
call_int_hook(cred_prepare, ...
if (cred)
create_user_ns(cred)
We noticed that error codes were not propagated from this hook and
introduced a patch [1] to propagate those errors.
The discussion notes that security_prepare_creds()
is not appropriate for MAC policies, and instead the hook is
meant for LSM authors to prepare credentials for mutation. [2]
Ultimately, we concluded that a better course of action is to introduce
a new security hook for LSM authors. [3]
This patch set first introduces a new security_create_user_ns() function
and create_user_ns LSM hook, then marks the hook as sleepable in BPF.
Why restrict this hook to user namespaces? It seems that an LSM that
chooses to preform controls on user namespaces may want to do so for
network namespaces as well.
IIRC, CLONE_NEWUSER is the only namespace flag that does not require
CAP_SYS_ADMIN. There is a security use case to prevent this namespace
from being created within an unprivileged environment. I'm not opposed
to a more generic hook, but I don't currently have a use case to block
any others. We can also say the same is true for the other namespaces:
add this generic security function to these too.
I'm curious what others think about this too.
Also, the hook seems backwards. You should
decide if the creation of the namespace is allowed before you create it.
Passing the new namespace to a function that checks to see creating a
namespace is allowed doesn't make a lot off sense.
I think having more context to a security hook is a good thing. I
believe you brought up in the previous discussions that you'd like to
use this hook for xattr purposes. Doesn't that require a namespace?
Links:
1.
https://lore.kernel.org/all/20220608150942.776446-1-fred@xxxxxxxxxxxxxx/
2.
https://lore.kernel.org/all/87y1xzyhub.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
3.
https://lore.kernel.org/all/9fe9cd9f-1ded-a179-8ded-5fde8960a586@xxxxxxxxxxxxxx/
Frederick Lawler (2):
security, lsm: Introduce security_create_user_ns()
bpf-lsm: Make bpf_lsm_create_user_ns() sleepable
include/linux/lsm_hook_defs.h | 2 ++
include/linux/lsm_hooks.h | 5 +++++
include/linux/security.h | 8 ++++++++
kernel/bpf/bpf_lsm.c | 1 +
kernel/user_namespace.c | 5 +++++
security/security.c | 6 ++++++
6 files changed, 27 insertions(+)
--
2.30.2
