powerpc/watchpoints: Disable preemption in thread_change_pc()

[ Upstream commit cc879ab ]

thread_change_pc() uses CPU local data, so must be protected from
swapping CPUs while it is reading the breakpoint struct.

The error is more noticeable after 1e60f35 ("powerpc/watchpoints:
Track perf single step directly on the breakpoint"), which added an
unconditional __this_cpu_read() call in thread_change_pc(). However the
existing __this_cpu_read() that runs if a breakpoint does need to be
re-inserted has the same issue.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230829063457.54157-2-bgray@linux.ibm.com
Signed-off-by: Sasha Levin <sashal@kernel.org>

arch/powerpc/kernel/hw_breakpoint.c

@@ -505,11 +505,13 @@ void thread_change_pc(struct task_struct *tsk, struct pt_regs *regs)
 	struct arch_hw_breakpoint *info;
 	int i;
 
+	preempt_disable();
+
 	for (i = 0; i < nr_wp_slots(); i++) {
 		if (unlikely(tsk->thread.last_hit_ubp[i]))
 			goto reset;
 	}
-	return;
+	goto out;
 
 reset:
 	regs_set_return_msr(regs, regs->msr & ~MSR_SE);
@@ -518,6 +520,9 @@ void thread_change_pc(struct task_struct *tsk, struct pt_regs *regs)
 		__set_breakpoint(i, info);
 		tsk->thread.last_hit_ubp[i] = NULL;
 	}
+
+out:
+	preempt_enable();
 }
 
 static bool is_larx_stcx_instr(int type)