[POWERPC] kprobes: Fix possible system crash during out-of-line single-stepping

- On archs that have no-exec support, we vmalloc() a executable scratch
area of PAGE_SIZE and divide it up into an array of slots of maximum
instruction size for that arch
- On a kprobe registration, the original instruction is copied to the
first available free slot, so if multiple kprobes are registered, chances
are, they get contiguous slots
- On POWER4, due to not having coherent icaches, we could hit a situation
where a probe that is registered on one processor, is hit immediately on
another. This second processor could have fetched the stream of text from
the out-of-line single-stepping area *before* the probe registration
completed, possibly due to an earlier (and a different) kprobe hit and
hence would see stale data at the slot.

Executing such an arbitrary instruction lead to a problem as reported
in LTC bugzilla 23555.

The correct solution is to call flush_icache_range() as soon as the
instruction is copied for out-of-line single-stepping, so the correct
instruction is seen on all processors.

Thanks to Will Schmidt who tracked this down.
Signed-off-by: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
Acked-by: Will Schmidt <will_schmidt@vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/kprobes.c

@@ -61,6 +61,8 @@ int __kprobes arch_prepare_kprobe(struct kprobe *p)
 	if (!ret) {
 		memcpy(p->ainsn.insn, p->addr, MAX_INSN_SIZE * sizeof(kprobe_opcode_t));
 		p->opcode = *p->addr;
+		flush_icache_range((unsigned long)p->ainsn.insn,
+			(unsigned long)p->ainsn.insn + sizeof(kprobe_opcode_t));
 	}
 
 	return ret;