[RFC PATCH V2 1/4] capabilities: use macros to make the logic easier to follow and verify

[Richard Guy Briggs]

This change is intended to be logic-neutral and simply make the logic easier to
read in natural language and verify without getting distracted by details.

Signed-off-by: Richard Guy Briggs <rgb at redhat.com>
---
 security/commoncap.c |   53 ++++++++++++++++++++++++++++++++-----------------
 1 files changed, 34 insertions(+), 19 deletions(-)

diff --git a/security/commoncap.c b/security/commoncap.c
index 78b3783..9520f0a 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -497,6 +497,16 @@ int cap_bprm_set_creds(struct linux_binprm *bprm)
 	int ret;
 	kuid_t root_uid;
 
+#define SROOT !issecure(SECURE_NOROOT) /* root is special */
+#define RROOT uid_eq(new->uid, root_uid) /* real root */
+#define EROOT uid_eq(new->euid, root_uid) /* effective root */
+#define SETUIDROOT !RROOT && EROOT /* set uid root */
+#define SUID !uid_eq(new->euid, old->uid) /* set uid */
+#define SGID !gid_eq(new->egid, old->gid) /* set gid */
+#define pPADD !cap_issubset(new->cap_permitted, old->cap_permitted) /* process permitted capabilities have been added */
+#define pESET !cap_issubset(new->cap_effective, new->cap_ambient) /* process effective capabilities have been set */
+#define pEALL cap_issubset(CAP_FULL_SET, new->cap_effective) /* process effective capabilities are full set */
+#define pAADD !cap_issubset(new->cap_ambient, old->cap_ambient) /* process ambient capabilities have been added */
 	if (WARN_ON(!cap_ambient_invariant_ok(old)))
 		return -EPERM;
 
@@ -507,13 +517,13 @@ int cap_bprm_set_creds(struct linux_binprm *bprm)
 
 	root_uid = make_kuid(new->user_ns, 0);
 
-	if (!issecure(SECURE_NOROOT)) {
+	if (SROOT) {
 		/*
 		 * If the legacy file capability is set, then don't set privs
 		 * for a setuid root binary run by a non-root user.  Do set it
 		 * for a root user just to cause least surprise to an admin.
 		 */
-		if (has_cap && !uid_eq(new->uid, root_uid) && uid_eq(new->euid, root_uid)) {
+		if (has_cap && SETUIDROOT) {
 			warn_setuid_and_fcaps_mixed(bprm->filename);
 			goto skip;
 		}
@@ -521,33 +531,32 @@ int cap_bprm_set_creds(struct linux_binprm *bprm)
 		 * To support inheritance of root-permissions and suid-root
 		 * executables under compatibility mode, we override the
 		 * capability sets for the file.
-		 *
-		 * If only the real uid is 0, we do not set the effective bit.
 		 */
-		if (uid_eq(new->euid, root_uid) || uid_eq(new->uid, root_uid)) {
+		if (EROOT || RROOT) {
 			/* pP' = (cap_bset & ~0) | (pI & ~0) */
 			new->cap_permitted = cap_combine(old->cap_bset,
 							 old->cap_inheritable);
 		}
-		if (uid_eq(new->euid, root_uid))
+		/*
+		 * If only the real uid is root, we do not set the effective bit.
+		 */
+		if (EROOT)
 			effective = true;
 	}
 skip:
 
 	/* if we have fs caps, clear dangerous personality flags */
-	if (!cap_issubset(new->cap_permitted, old->cap_permitted))
+	if (pPADD)
 		bprm->per_clear |= PER_CLEAR_ON_SETID;
 
+	is_setid = SUID || SGID;
 
 	/* Don't let someone trace a set[ug]id/setpcap binary with the revised
 	 * credentials unless they have the appropriate permit.
 	 *
 	 * In addition, if NO_NEW_PRIVS, then ensure we get no new privs.
 	 */
-	is_setid = !uid_eq(new->euid, old->uid) || !gid_eq(new->egid, old->gid);
-
-	if ((is_setid ||
-	     !cap_issubset(new->cap_permitted, old->cap_permitted)) &&
+	if ((is_setid || pPADD) &&
 	    ((bprm->unsafe & ~LSM_UNSAFE_PTRACE) ||
 	     !ptracer_capable(current, new->user_ns))) {
 		/* downgrade; they get no more than they had, and maybe less */
@@ -599,14 +608,10 @@ int cap_bprm_set_creds(struct linux_binprm *bprm)
 	 * Number 1 above might fail if you don't have a full bset, but I think
 	 * that is interesting information to audit.
 	 */
-	if (!cap_issubset(new->cap_effective, new->cap_ambient)) {
-		if (!cap_issubset(CAP_FULL_SET, new->cap_effective) ||
-		    !uid_eq(new->euid, root_uid) || !uid_eq(new->uid, root_uid) ||
-		    issecure(SECURE_NOROOT)) {
-			ret = audit_log_bprm_fcaps(bprm, new, old);
-			if (ret < 0)
-				return ret;
-		}
+	if (pESET && (!pEALL || !EROOT || !RROOT || !SROOT) ) {
+		ret = audit_log_bprm_fcaps(bprm, new, old);
+		if (ret < 0)
+			return ret;
 	}
 
 	new->securebits &= ~issecure_mask(SECURE_KEEP_CAPS);
@@ -615,6 +620,16 @@ int cap_bprm_set_creds(struct linux_binprm *bprm)
 		return -EPERM;
 
 	return 0;
+#undef SROOT
+#undef RROOT
+#undef EROOT
+#undef SETUIDROOT
+#undef SUID
+#undef SGID
+#undef pPADD
+#undef pESET
+#undef pEALL
+#undef pAADD
 }
 
 /**
-- 
1.7.1

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