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

kernel/kprobes.c

@@ -675,9 +675,18 @@ static struct notifier_block kprobe_exceptions_nb = {
 	.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;