Re: SELinux: How to split permissions for keys?

From: Stephen Smalley
Date: Mon Feb 03 2020 - 08:12:13 EST

On 2/2/20 2:30 PM, Richard Haines wrote:
On Thu, 2020-01-23 at 15:35 -0500, Stephen Smalley wrote:
On 1/23/20 10:46 AM, Stephen Smalley wrote:
On 1/23/20 10:12 AM, David Howells wrote:
Hi Stephen,

I have patches to split the permissions that are used for keys to
make
them a
bit finer grained and easier to use - and also to move to ACLs
rather
than
fixed masks. See patch "keys: Replace uid/gid/perm permissions
checking with
an ACL" here:

https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=keys-acl


However, I may not have managed the permission mask
transformation inside
SELinux correctly. Could you lend an eyeball? The change to
the
permissions
model is as follows:

The SETATTR permission is split to create two new
permissions:
(1) SET_SECURITY - which allows the key's owner, group and
ACL
to be
changed and a restriction to be placed on a keyring.
(2) REVOKE - which allows a key to be revoked.
The SEARCH permission is split to create:
(1) SEARCH - which allows a keyring to be search and a key
to be
found.
(2) JOIN - which allows a keyring to be joined as a
session
keyring.
(3) INVAL - which allows a key to be invalidated.
The WRITE permission is also split to create:
(1) WRITE - which allows a key's content to be altered and
links
to be
added, removed and replaced in a keyring.
(2) CLEAR - which allows a keyring to be cleared
completely.
This is
split out to make it possible to give just this to an
administrator.
(3) REVOKE - see above.

The change to SELinux is attached below.

Should the split be pushed down into the SELinux policy rather
than
trying to
calculate it?

My understanding is that you must provide full backward
compatibility
with existing policies; hence, you must ensure that you always
check the
same SELinux permission(s) for the same operation when using an
existing
policy.

In order to support finer-grained distinctions in SELinux with
future
policies, you can define a new SELinux policy capability along with
the
new permissions, and if the policy capability is enabled in the
policy,
check the new permissions rather than the old ones. A recent
example of
adding a new policy capability and using it can be seen in:
https://lore.kernel.org/selinux/20200116194530.8696-1-jeffv@xxxxxxxxxx/T/#u
although that patch was rejected for other reasons.

Another example was when we introduced fine-grained distinctions
for all
network address families, commit
da69a5306ab92e07224da54aafee8b1dccf024f6.

The new policy capability also needs to be defined in libsepol for
use
by the policy compiler; an example can be seen in:
https://lore.kernel.org/selinux/20170714164801.6346-1-sds@xxxxxxxxxxxxx/

Then future policies can declare the policy capability when they
are
ready to start using the new permissions instead of the old.

Thanks,
David
---
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 116b4d644f68..c8db5235b01f 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -6556,6 +6556,7 @@ static int
selinux_key_permission(key_ref_t
key_ref,
{
struct key *key;
struct key_security_struct *ksec;
+ unsigned oldstyle_perm;
u32 sid;
/* if no specific permissions are requested, we skip the
@@ -6564,13 +6565,26 @@ static int
selinux_key_permission(key_ref_t
key_ref,
if (perm == 0)
return 0;
+ oldstyle_perm = perm & (KEY_NEED_VIEW | KEY_NEED_READ |
KEY_NEED_WRITE |
+ KEY_NEED_SEARCH | KEY_NEED_LINK);
+ if (perm & KEY_NEED_SETSEC)
+ oldstyle_perm |= OLD_KEY_NEED_SETATTR;
+ if (perm & KEY_NEED_INVAL)
+ oldstyle_perm |= KEY_NEED_SEARCH;
+ if (perm & KEY_NEED_REVOKE && !(perm &
OLD_KEY_NEED_SETATTR))
+ oldstyle_perm |= KEY_NEED_WRITE;
+ if (perm & KEY_NEED_JOIN)
+ oldstyle_perm |= KEY_NEED_SEARCH;
+ if (perm & KEY_NEED_CLEAR)
+ oldstyle_perm |= KEY_NEED_WRITE;
+

I don't know offhand if this ensures that the same SELinux
permission is
always checked as it would have been previously for the same
operation+arguments. That's what you have to preserve for
existing
policies.

As Richard pointed out in his email, your key-acl series replaces
two
different old permissions (LINK, SEARCH) with a single permission
(JOIN)
in different callers, so by the time we reach the SELinux hook we
cannot
map it back unambiguously and provide full backward
compatibility. The
REVOKE case also seems fragile although there you seem to distinguish
by
sometimes passing in OLD_KEY_NEED_SETATTR and sometimes not? You'll
have to fix the JOIN case to avoid userspace breakage.

You may want to go ahead and explicitly translate all of the
KEY_NEED
permissions to SELinux permissions rather than passing the key
permissions directly here to avoid requiring that the values always
match. The SELinux permission symbols are of the form
CLASS__PERMISSION
(NB double underscore), e.g. KEY__SETATTR, generated automatically
from
the security/selinux/include/classmap.h tables to the
security/selinux/av_permissions.h generated header. Most hooks
perform
such translation, e.g. file_mask_to_av(). You will almost certainly
need to do this if/when you introduce support for the new permissions
to
SELinux.


This problem has now been fixed in [1].
It passes the current selinux-test-suite (except test/fs_filesystem
regression).

As the fix now includes a new 'key_perms' policy capability to allow
use of the extended key permissions, I've updated libsepol and the
selinux-testsuite test/keys to test these.

I'll submit two RFC patches that will allow [1] to be tested with
'key_perms' true or false.

[1]
https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/commit/?h=keys-next

Was that kernel patch ever posted to selinux list and/or the selinux kernel maintainers? I don't recall seeing it. If not, please send it to the selinux list for review; at least one selinux maintainer should ack it before it gets accepted into any other tree.