kprobes: fix a memory leak in function pre_handler_kretprobe()

In function pre_handler_kretprobe(), the allocated kretprobe_instance
object will get leaked if the entry_handler callback returns non-zero.
This may cause all the preallocated kretprobe_instance objects exhausted.

This issue can be reproduced by changing
samples/kprobes/kretprobe_example.c to probe "mutex_unlock".  And the fix
is straightforward: just put the allocated kretprobe_instance object back
onto the free_instances list.

[[email protected]: use raw_spin_lock/unlock]
Signed-off-by: Jiang Liu <[email protected]>
Acked-by: Jim Keniston <[email protected]>
Acked-by: Ananth N Mavinakayanahalli <[email protected]>
Cc: Masami Hiramatsu <[email protected]>
Cc: Anil S Keshavamurthy <[email protected]>
Cc: "David S. Miller" <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: jiangliu

kernel/kprobes.c

@@ -1673,8 +1673,12 @@ static int __kprobes pre_handler_kretprobe(struct kprobe *p,
 		ri->rp = rp;
 		ri->task = current;
 
-		if (rp->entry_handler && rp->entry_handler(ri, regs))
+		if (rp->entry_handler && rp->entry_handler(ri, regs)) {
+			raw_spin_lock_irqsave(&rp->lock, flags);
+			hlist_add_head(&ri->hlist, &rp->free_instances);
+			raw_spin_unlock_irqrestore(&rp->lock, flags);
 			return 0;
+		}
 
 		arch_prepare_kretprobe(ri, regs);