MIPS: Disable usermode switching of the FR bit for MIPS R5 CPUs.

Currently the kernel will always use the FR=0 register model for O32.  If
an O32 application did enable FR=1 mode, some data from another application
might be leaked in the extra registers becoming visible.

Iow, this patch is meant to make the kernel MIPS R5 tolerant but leaves
proper MIPS R5 support to a future patchset.
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

arch/mips/kernel/cpu-probe.c

@@ -291,6 +291,17 @@ static inline unsigned int decode_config4(struct cpuinfo_mips *c)
 	return config4 & MIPS_CONF_M;
 }
 
+static inline unsigned int decode_config5(struct cpuinfo_mips *c)
+{
+	unsigned int config5;
+
+	config5 = read_c0_config5();
+	config5 &= ~MIPS_CONF5_UFR;
+	write_c0_config5(config5);
+
+	return config5 & MIPS_CONF_M;
+}
+
 static void decode_configs(struct cpuinfo_mips *c)
 {
 	int ok;
@@ -311,6 +322,8 @@ static void decode_configs(struct cpuinfo_mips *c)
 		ok = decode_config3(c);
 	if (ok)
 		ok = decode_config4(c);
+	if (ok)
+		ok = decode_config5(c);
 
 	mips_probe_watch_registers(c);