[kernel-hardening] [RFC PATCH 1/2] security, capabilities: create CAP_TRUSTED

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From: Nicolas Belouin <redacted>
Message-ID: [off-list ref]



On October 21, 2017 7:25:21 PM GMT+02:00, Casey Schaufler [off-list ref] wrote:

On 10/21/2017 6:45 AM, Nicolas Belouin wrote:

quoted

with CAP_SYS_ADMIN being bloated, the usefulness of using it to
flag a process to be entrusted for e.g reading and writing trusted
xattr is near zero.
CAP_TRUSTED aims to provide userland with a way to mark a process as
entrusted to do specific (not specially admin-centered) actions. It
would for example allow a process to red/write the trusted xattrs.

Please explain how this is different from CAP_MAC_ADMIN in
any existing use case. If it is significantly different, how
would the two interact?

From my point of view, CAP_MAC_ADMIN allows one to read/write security xattrs, those are meant to describe security policies. As far as I know of, trusted xattrs are intended for a privileged process to read or write arbitrary data. I don't have any real world example in mind that use trusted xattrs, but I'll try to find one.

quoted

Signed-off-by: Nicolas Belouin <redacted>
---
 include/uapi/linux/capability.h     | 6 +++++-
 security/selinux/include/classmap.h | 5 +++--
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/include/uapi/linux/capability.h

b/include/uapi/linux/capability.h

quoted

index ce230aa6d928..27e457b93c84 100644

--- a/include/uapi/linux/capability.h
+++ b/include/uapi/linux/capability.h

@@ -369,7 +369,11 @@ struct vfs_ns_cap_data {
 
 #define CAP_SYS_MOUNT		38
 
-#define CAP_LAST_CAP         CAP_SYS_MOUNT
+/* Allow read/write trusted xattr */
+
+#define CAP_TRUSTED		39
+
+#define CAP_LAST_CAP         CAP_TRUSTED
 
 #define cap_valid(x) ((x) >= 0 && (x) <= CAP_LAST_CAP)
 

diff --git a/security/selinux/include/classmap.h

b/security/selinux/include/classmap.h

quoted

index a873dce97fd5..f5dc8e109f5a 100644

--- a/security/selinux/include/classmap.h
+++ b/security/selinux/include/classmap.h

@@ -24,9 +24,10 @@
 	    "audit_control", "setfcap"
 
 #define COMMON_CAP2_PERMS  "mac_override", "mac_admin", "syslog", \
-		"wake_alarm", "block_suspend", "audit_read", "sys_mount"
+		"wake_alarm", "block_suspend", "audit_read", "sys_mount", \
+		"trusted"
 
-#if CAP_LAST_CAP > CAP_SYS_MOUNT
+#if CAP_LAST_CAP > CAP_TRUSTED
 #error New capability defined, please update COMMON_CAP2_PERMS.
 #endif
 

Nicolas
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