- When executing FreeBSD/amd64 binaries from FreeBSD/i386 or Linux/i386

processes, clear PCB_32BIT and PCB_GS32BIT bits [1].

- Reread the fs and gs bases from the msr unconditionally, not believing
  the values in pcb_fsbase and pcb_gsbase, since usermode may reload
  segment registers, invalidating the cache. [2].

Both problems resulted in the wrong fs base, causing wrong tls pointer
be dereferenced in the usermode.

Reported and tested by:	Vyacheslav Bocharov <adeepv at gmail com> [1]
Reported by:	Bernd Walter <ticsoat cicely7 cicely de>,
	Artem Belevich <fbsdlist at src cx>[2]
Reviewed by:	peter
MFC after:	3 days

sys/amd64/amd64/cpu_switch.S

@@ -109,8 +109,24 @@ ENTRY(cpu_switch)
 	movq	%rsp,PCB_RSP(%r8)
 	movq	%rbx,PCB_RBX(%r8)
 	movq	%rax,PCB_RIP(%r8)
-	movq	PCB_FSBASE(%r8),%r9
-	movq	PCB_GSBASE(%r8),%r10
+
+	/*
+	 * Reread fs and gs bases. Explicit fs segment register load
+	 * by the usermode code may change actual fs base without
+	 * updating pcb_{fs,gs}base.
+	 *
+	 * %rdx still contains the mtx, save %rdx around rdmsr.
+	 */
+	movq	%rdx,%r11
+	movl	$MSR_FSBASE,%ecx
+	rdmsr
+	shlq	$32,%rdx
+	leaq	(%rax,%rdx),%r9
+	movl	$MSR_KGSBASE,%ecx
+	rdmsr
+	shlq	$32,%rdx
+	leaq	(%rax,%rdx),%r10
+	movq	%r11,%rdx
 
 	testl	$PCB_32BIT,PCB_FLAGS(%r8)
 	jnz	store_seg

sys/amd64/amd64/machdep.c

@@ -734,6 +734,7 @@ exec_setregs(td, entry, stack, ps_strings)
 	pcb->pcb_fsbase = 0;
 	pcb->pcb_gsbase = 0;
 	critical_exit();
+	pcb->pcb_flags &= ~(PCB_32BIT | PCB_GS32BIT);
 	load_ds(_udatasel);
 	load_es(_udatasel);
 	load_fs(_udatasel);

sys/amd64/ia32/ia32_signal.c

@@ -742,5 +742,6 @@ ia32_setregs(td, entry, stack, ps_strings)
 
 	/* Return via doreti so that we can change to a different %cs */
 	pcb->pcb_flags |= PCB_FULLCTX | PCB_32BIT;
+	pcb->pcb_flags &= ~PCB_GS32BIT;
 	td->td_retval[1] = 0;
 }