ethersubr: Make the mac address generation more robust

If we create two (vnet) jails and create a bridge interface in each we end up
with the same mac address on both bridge interfaces.
These very often conflicts, resulting in same mac address in both jails.

Mitigate this problem by including the jail name in the mac address.

Reviewed by:	kevans, melifaro
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D24383

sys/kern/kern_jail.c

@@ -2920,6 +2920,15 @@ getcredhostid(struct ucred *cred, unsigned long *hostid)
 	mtx_unlock(&cred->cr_prison->pr_mtx);
 }
 
+void
+getjailname(struct ucred *cred, char *name, size_t len)
+{
+
+	mtx_lock(&cred->cr_prison->pr_mtx);
+	strlcpy(name, cred->cr_prison->pr_name, len);
+	mtx_unlock(&cred->cr_prison->pr_mtx);
+}
+
 #ifdef VIMAGE
 /*
  * Determine whether the prison represented by cred owns

sys/net/if_ethersubr.c

@@ -1419,27 +1419,39 @@ ether_8021q_frame(struct mbuf **mp, struct ifnet *ife, struct ifnet *p,
 
 /*
  * Allocate an address from the FreeBSD Foundation OUI.  This uses a
- * cryptographic hash function on the containing jail's UUID and the interface
- * name to attempt to provide a unique but stable address.  Pseudo-interfaces
- * which require a MAC address should use this function to allocate
- * non-locally-administered addresses.
+ * cryptographic hash function on the containing jail's name, UUID and the
+ * interface name to attempt to provide a unique but stable address.
+ * Pseudo-interfaces which require a MAC address should use this function to
+ * allocate non-locally-administered addresses.
  */
 void
 ether_gen_addr(struct ifnet *ifp, struct ether_addr *hwaddr)
 {
-#define	ETHER_GEN_ADDR_BUFSIZ	HOSTUUIDLEN + IFNAMSIZ + 2
 	SHA1_CTX ctx;
-	char buf[ETHER_GEN_ADDR_BUFSIZ];
+	char *buf;
 	char uuid[HOSTUUIDLEN + 1];
 	uint64_t addr;
 	int i, sz;
 	char digest[SHA1_RESULTLEN];
+	char jailname[MAXHOSTNAMELEN];
 
 	getcredhostuuid(curthread->td_ucred, uuid, sizeof(uuid));
-	sz = snprintf(buf, ETHER_GEN_ADDR_BUFSIZ, "%s-%s", uuid, ifp->if_xname);
+	/* If each (vnet) jail would also have a unique hostuuid this would not
+	 * be necessary. */
+	getjailname(curthread->td_ucred, jailname, sizeof(jailname));
+	sz = asprintf(&buf, M_TEMP, "%s-%s-%s", uuid, if_name(ifp),
+	    jailname);
+	if (sz < 0) {
+		/* Fall back to a random mac address. */
+		arc4rand(hwaddr, sizeof(*hwaddr), 0);
+		hwaddr->octet[0] = 0x02;
+		return;
+	}
+
 	SHA1Init(&ctx);
 	SHA1Update(&ctx, buf, sz);
 	SHA1Final(digest, &ctx);
+	free(buf, M_TEMP);
 
 	addr = ((digest[0] << 16) | (digest[1] << 8) | digest[2]) &
 	    OUI_FREEBSD_GENERATED_MASK;

sys/sys/jail.h

@@ -382,6 +382,7 @@ void getcredhostname(struct ucred *, char *, size_t);
 void getcreddomainname(struct ucred *, char *, size_t);
 void getcredhostuuid(struct ucred *, char *, size_t);
 void getcredhostid(struct ucred *, unsigned long *);
+void getjailname(struct ucred *cred, char *name, size_t len);
 void prison0_init(void);
 int prison_allow(struct ucred *, unsigned);
 int prison_check(struct ucred *cred1, struct ucred *cred2);