jprobes: make jprobes a little safer for users

I realise jprobes are a razor-blades-included type of interface, but that
doesn't mean we can't try and make them safer to use. This guy I know once
wrote code like this:

struct jprobe jp = { .kp.symbol_name = "foo", .entry = "jprobe_foo" };

And then his kernel exploded. Oops.

This patch adds an arch hook, arch_deref_entry_point() (I don't like it
either) which takes the void * in a struct jprobe, and gives back the text
address that it represents.

We can then use that in register_jprobe() to check that the entry point we're
passed is actually in the kernel text, rather than just some random value.

Signed-off-by: Michael Ellerman <[e-mail]>
Cc: Prasanna S Panchamukhi <[e-mail]>
Acked-by: Ananth N Mavinakayanahalli <[e-mail]>
Cc: Anil S Keshavamurthy <[e-mail]>
Cc: David S. Miller <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/ia64/kernel/kprobes.c

···
 	return;
 }
 
+unsigned long arch_deref_entry_point(void *entry)
+{
+	return ((struct fnptr *)entry)->ip;
+}
+
 int __kprobes setjmp_pre_handler(struct kprobe *p, struct pt_regs *regs)
 {
 	struct jprobe *jp = container_of(p, struct jprobe, kp);
-	unsigned long addr = ((struct fnptr *)(jp->entry))->ip;
+	unsigned long addr = arch_deref_entry_point(jp->entry);
 	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
 	struct param_bsp_cfm pa;
 	int bytes;

arch/powerpc/kernel/kprobes.c

···
 	return ret;
 }
 
+#ifdef CONFIG_PPC64
+unsigned long arch_deref_entry_point(void *entry)
+{
+	return (unsigned long)(((func_descr_t *)entry)->entry);
+}
+#endif
+
 int __kprobes setjmp_pre_handler(struct kprobe *p, struct pt_regs *regs)
 {
 	struct jprobe *jp = container_of(p, struct jprobe, kp);
···
 	memcpy(&kcb->jprobe_saved_regs, regs, sizeof(struct pt_regs));
 
 	/* setup return addr to the jprobe handler routine */
+	regs->nip = arch_deref_entry_point(jp->entry);
 #ifdef CONFIG_PPC64
-	regs->nip = (unsigned long)(((func_descr_t *)jp->entry)->entry);
 	regs->gpr[2] = (unsigned long)(((func_descr_t *)jp->entry)->toc);
-#else
-	regs->nip = (unsigned long)jp->entry;
 #endif
 
 	return 1;

include/linux/kprobes.h

···
 int register_jprobe(struct jprobe *p);
 void unregister_jprobe(struct jprobe *p);
 void jprobe_return(void);
+unsigned long arch_deref_entry_point(void *);
 
 int register_kretprobe(struct kretprobe *rp);
 void unregister_kretprobe(struct kretprobe *rp);

kernel/kprobes.c

···
 	.priority = 0x7fffffff /* we need to be notified first */
 };
 
+unsigned long __weak arch_deref_entry_point(void *entry)
+{
+	return (unsigned long)entry;
+}
 
 int __kprobes register_jprobe(struct jprobe *jp)
 {
+	unsigned long addr = arch_deref_entry_point(jp->entry);
+
+	if (!kernel_text_address(addr))
+		return -EINVAL;
+
 	/* Todo: Verify probepoint is a function entry point */
 	jp->kp.pre_handler = setjmp_pre_handler;
 	jp->kp.break_handler = longjmp_break_handler;