Index: sys/amd64/amd64/vm_machdep.c
===================================================================
--- sys/amd64/amd64/vm_machdep.c
+++ sys/amd64/amd64/vm_machdep.c
@@ -569,12 +569,14 @@
 	 * Those not loaded individually below get their default
 	 * values here.
 	 */
-	if (td0 == curthread)
-		update_pcb_bases(td0->td_pcb);
 	bcopy(td0->td_pcb, pcb2, sizeof(*pcb2));
 	clear_pcb_flags(pcb2, PCB_FPUINITDONE | PCB_USERFPUINITDONE |
 	    PCB_KERNFPU);
 	pcb2->pcb_save = get_pcb_user_save_pcb(pcb2);
+
+	/* Reset the fs and gs bases. */
+	pcb2->pcb_fsbase = 0;
+	pcb2->pcb_gsbase = 0;
 	set_pcb_flags_raw(pcb2, PCB_FULL_IRET);
 
 	/*
@@ -603,7 +605,6 @@
 	 * If we didn't copy the pcb, we'd need to do the following registers:
 	 * pcb2->pcb_dr*:	cloned above.
 	 * pcb2->pcb_onfault:	cloned above (always NULL here?).
-	 * pcb2->pcb_[fg]sbase: cloned above
 	 */
 
 	/* Setup to release spin count in fork_exit(). */
Index: sys/i386/i386/sys_machdep.c
===================================================================
--- sys/i386/i386/sys_machdep.c
+++ sys/i386/i386/sys_machdep.c
@@ -108,7 +108,8 @@
 	fill_based_sd(&sd, base);
 	critical_enter();
 	td->td_pcb->pcb_fsd = sd;
-	PCPU_GET(fsgs_gdt)[0] = sd;
+	if (td == curthread)
+		PCPU_GET(fsgs_gdt)[0] = sd;
 	critical_exit();
 }
 
@@ -120,7 +121,8 @@
 	fill_based_sd(&sd, base);
 	critical_enter();
 	td->td_pcb->pcb_gsd = sd;
-	PCPU_GET(fsgs_gdt)[1] = sd;
+	if (td == curthread)
+		PCPU_GET(fsgs_gdt)[1] = sd;
 	critical_exit();
 }
 
Index: sys/i386/i386/vm_machdep.c
===================================================================
--- sys/i386/i386/vm_machdep.c
+++ sys/i386/i386/vm_machdep.c
@@ -438,6 +438,10 @@
 	    PCB_KERNNPX);
 	pcb2->pcb_save = get_pcb_user_save_pcb(pcb2);
 
+	/* Reset the fs and gs bases. */
+	set_fsbase(td, 0);
+	set_gsbase(td, 0);
+
 	/*
 	 * Create a new fresh stack for the new thread.
 	 */
@@ -461,14 +465,13 @@
 	pcb2->pcb_esp = (int)td->td_frame - sizeof(void *); /* trampoline arg */
 	pcb2->pcb_ebx = (int)td;			    /* trampoline arg */
 	pcb2->pcb_eip = (int)fork_trampoline + setidt_disp;
-	pcb2->pcb_gs = rgs();
+	pcb2->pcb_gs = _udatasel;
 	/*
 	 * If we didn't copy the pcb, we'd need to do the following registers:
 	 * pcb2->pcb_cr3:	cloned above.
 	 * pcb2->pcb_dr*:	cloned above.
 	 * pcb2->pcb_flags:	cloned above.
 	 * pcb2->pcb_onfault:	cloned above (always NULL here?).
-	 * pcb2->pcb_gs:	cloned above.
 	 * pcb2->pcb_ext:	cleared below.
 	 */
 	pcb2->pcb_ext = NULL;