[RFC] Implement ambient capability set.

[Christoph Lameter]

An attempt to implement this. Probably missing some fine points:

Subject: [capabilities] Implement ambient capability set.

DRAFT -- untested -- DRAFT

Implement an ambient capabilty set to allow capabilties
to be inherited with unix semantics used also for other
attributes.

Implements PR_CAP_AMBIENT. The second argument to prctl
is a the capability number and the third the desired state.
0 for off. Otherwise on.

Serge:
A new capability set, pA, is empty by default.  You can
add bits to it using prctl if ns_capable(CAP_SETPCAP) and
all the new bits are in your pE.  Once set, they stay until
they are removed using prctl.  At exec, pA' = pA, and
fI |= pA (after reading fI from disk but before
calculating pI').

Since the ambient caps "stay on" cap_inheritable does not
really matter anymore. Simply set the permitted caps when
the ambient cap is set.

Signed-off-by: Christoph Lameter <cl@xxxxxxxxx>

Index: linux/security/commoncap.c
===================================================================
--- linux.orig/security/commoncap.c	2015-02-04 09:44:25.000000000 -0600
+++ linux/security/commoncap.c	2015-02-04 12:48:44.100471600 -0600
@@ -353,7 +353,7 @@ static inline int bprm_caps_from_vfs_cap
 		/*
 		 * pP' = (X & fP) | (pI & fI)
 		 */
-		new->cap_permitted.cap[i] =
+		new->cap_permitted.cap[i] = current_cred()->cap_ambient.cap[i] |
 			(new->cap_bset.cap[i] & permitted) |
 			(new->cap_inheritable.cap[i] & inheritable);

@@ -577,6 +577,7 @@ skip:
 	}

 	new->securebits &= ~issecure_mask(SECURE_KEEP_CAPS);
+	new->cap_ambient = old->cap_ambient;
 	return 0;
 }

@@ -933,6 +934,20 @@ int cap_task_prctl(int option, unsigned
 			new->securebits &= ~issecure_mask(SECURE_KEEP_CAPS);
 		return commit_creds(new);

+	case PR_CAP_AMBIENT:
+		if (!ns_capable(current_user_ns(), CAP_SETPCAP))
+			return -EPERM;
+
+		if (!cap_valid(arg2))
+			return -EINVAL;
+
+		new =prepare_creds();
+		if (arg3 == 0)
+			cap_lower(new->cap_ambient, arg2);
+		else
+			cap_raise(new->cap_ambient, arg2);
+		return commit_creds(new);
+
 	default:
 		/* No functionality available - continue with default */
 		return -ENOSYS;
Index: linux/include/linux/cred.h
===================================================================
--- linux.orig/include/linux/cred.h	2015-02-04 09:39:46.000000000 -0600
+++ linux/include/linux/cred.h	2015-02-04 12:32:43.719846530 -0600
@@ -122,6 +122,7 @@ struct cred {
 	kernel_cap_t	cap_permitted;	/* caps we're permitted */
 	kernel_cap_t	cap_effective;	/* caps we can actually use */
 	kernel_cap_t	cap_bset;	/* capability bounding set */
+	kernel_cap_t	cap_ambient;	/* Ambient capability set */
 #ifdef CONFIG_KEYS
 	unsigned char	jit_keyring;	/* default keyring to attach requested
 					 * keys to */
Index: linux/include/uapi/linux/prctl.h
===================================================================
--- linux.orig/include/uapi/linux/prctl.h	2014-12-12 10:27:49.332800377 -0600
+++ linux/include/uapi/linux/prctl.h	2015-02-04 12:39:10.651205059 -0600
@@ -185,4 +185,7 @@ struct prctl_mm_map {
 #define PR_MPX_ENABLE_MANAGEMENT  43
 #define PR_MPX_DISABLE_MANAGEMENT 44

+/* Control the ambient capability set */
+#define PR_CAP_AMBIENT 45
+
 #endif /* _LINUX_PRCTL_H */
--
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