MIPS: Fix kernel crash for R6 in jump label branch function

commit 47c25036b60f27b86ab44b66a8861bcf81cde39b upstream.

Insert Branch instruction instead of NOP to make sure assembler don't
patch code in forbidden slot. In jump label function, it might
be possible to patch Control Transfer Instructions(CTIs) into
forbidden slot, which will generate Reserved Instruction exception
in MIPS release 6.
Signed-off-by: NArcher Yan <ayan@wavecomp.com>
Reviewed-by: NPaul Burton <paul.burton@mips.com>
[paul.burton@mips.com:
  - Add MIPS prefix to subject.
  - Mark for stable from v4.0, which introduced r6 support, onwards.]
Signed-off-by: NPaul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org
Cc: stable@vger.kernel.org # v4.0+
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/mips/include/asm/jump_label.h

@@ -21,15 +21,15 @@
 #endif
 
 #ifdef CONFIG_CPU_MICROMIPS
-#define NOP_INSN "nop32"
+#define B_INSN "b32"
 #else
-#define NOP_INSN "nop"
+#define B_INSN "b"
 #endif
 
 static __always_inline bool arch_static_branch(struct static_key *key, bool branch)
 {
-	asm_volatile_goto("1:\t" NOP_INSN "\n\t"
-		"nop\n\t"
+	asm_volatile_goto("1:\t" B_INSN " 2f\n\t"
+		"2:\tnop\n\t"
 		".pushsection __jump_table,  \"aw\"\n\t"
 		WORD_INSN " 1b, %l[l_yes], %0\n\t"
 		".popsection\n\t"