taskstats: don't allow duplicate entries in listener mode

Currently a single process may register exit handlers unlimited times.
It may lead to a bloated listeners chain and very slow process
terminations.

Eg after 10KK sent TASKSTATS_CMD_ATTR_REGISTER_CPUMASKs ~300 Mb of
kernel memory is stolen for the handlers chain and "time id" shows 2-7
seconds instead of normal 0.003.  It makes it possible to exhaust all
kernel memory and to eat much of CPU time by triggerring numerous exits
on a single CPU.

The patch limits the number of times a single process may register
itself on a single CPU to one.

One little issue is kept unfixed - as taskstats_exit() is called before
exit_files() in do_exit(), the orphaned listener entry (if it was not
explicitly deregistered) is kept until the next someone's exit() and
implicit deregistration in send_cpu_listeners().  So, if a process
registered itself as a listener exits and the next spawned process gets
the same pid, it would inherit taskstats attributes.

Signed-off-by: Vasiliy Kulikov <segooon@gmail.com>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 3 deletions

diff --git a/kernel/taskstats.c b/kernel/taskstats.c
index 9ffea360a778..fc0f22005417 100644
--- a/kernel/taskstats.c
+++ b/kernel/taskstats.c
@@ -285,16 +285,18 @@ ret:
 static int add_del_listener(pid_t pid, const struct cpumask *mask, int isadd)
 {
         struct listener_list *listeners;
-        struct listener *s, *tmp;
+        struct listener *s, *tmp, *s2;
         unsigned int cpu;
 
         if (!cpumask_subset(mask, cpu_possible_mask))
                 return -EINVAL;
 
+        s = NULL;
         if (isadd == REGISTER) {
                 for_each_cpu(cpu, mask) {
-                        s = kmalloc_node(sizeof(struct listener), GFP_KERNEL,
-                                         cpu_to_node(cpu));
+                        if (!s)
+                                s = kmalloc_node(sizeof(struct listener),
+                                                 GFP_KERNEL, cpu_to_node(cpu));
                         if (!s)
                                 goto cleanup;
                         s->pid = pid;
@@ -303,9 +305,16 @@ static int add_del_listener(pid_t pid, const struct cpumask *mask, int isadd)
 
                         listeners = &per_cpu(listener_array, cpu);
                         down_write(&listeners->sem);
+                        list_for_each_entry_safe(s2, tmp, &listeners->list, list) {
+                                if (s2->pid == pid)
+                                        goto next_cpu;
+                        }
                         list_add(&s->list, &listeners->list);
+                        s = NULL;
+next_cpu:
                         up_write(&listeners->sem);
                 }
+                kfree(s);
                 return 0;
         }