riscv: keep interrupts disabled for BREAKPOINT exception

Current riscv's kprobe handlers are run with both preemption and
interrupt enabled, this violates kprobe requirements. Fix this issue
by keeping interrupts disabled for BREAKPOINT exception.

Fixes: c22b0bcb ("riscv: Add kprobes supported")
Cc: stable@vger.kernel.org
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
[Palmer: add a comment]
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/kernel/entry.S

@@ -130,6 +130,9 @@ skip_context_tracking:
 	 */
 	andi t0, s1, SR_PIE
 	beqz t0, 1f
+	/* kprobes, entered via ebreak, must have interrupts disabled. */
+	li t0, EXC_BREAKPOINT
+	beq s4, t0, 1f
 #ifdef CONFIG_TRACE_IRQFLAGS
 	call trace_hardirqs_on
 #endif