powerpc/kprobes: Don't allow breakpoints on suffixes

Do not allow inserting breakpoints on the suffix of a prefix instruction
in kprobes.
Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200506034050.24806-28-jniethe5@gmail.com

arch/powerpc/kernel/kprobes.c

@@ -106,7 +106,9 @@ kprobe_opcode_t *kprobe_lookup_name(const char *name, unsigned int offset)
 int arch_prepare_kprobe(struct kprobe *p)
 {
 	int ret = 0;
+	struct kprobe *prev;
 	struct ppc_inst insn = ppc_inst_read((struct ppc_inst *)p->addr);
+	struct ppc_inst prefix = ppc_inst_read((struct ppc_inst *)(p->addr - 1));
 
 	if ((unsigned long)p->addr & 0x03) {
 		printk("Attempt to register kprobe at an unaligned address\n");
@@ -114,6 +116,17 @@ int arch_prepare_kprobe(struct kprobe *p)
 	} else if (IS_MTMSRD(insn) || IS_RFID(insn) || IS_RFI(insn)) {
 		printk("Cannot register a kprobe on rfi/rfid or mtmsr[d]\n");
 		ret = -EINVAL;
+	} else if (ppc_inst_prefixed(prefix)) {
+		printk("Cannot register a kprobe on the second word of prefixed instruction\n");
+		ret = -EINVAL;
+	}
+	preempt_disable();
+	prev = get_kprobe(p->addr - 1);
+	preempt_enable_no_resched();
+	if (prev &&
+	    ppc_inst_prefixed(ppc_inst_read((struct ppc_inst *)prev->ainsn.insn))) {
+		printk("Cannot register a kprobe on the second word of prefixed instruction\n");
+		ret = -EINVAL;
 	}
 
 	/* insn must be on a special executable page on ppc64.  This is