oom: fix the unsafe usage of badness() in proc_oom_score()

proc_oom_score(task) has a reference to task_struct, but that is all.
If this task was already released before we take tasklist_lock

	- we can't use task->group_leader, it points to nowhere

	- it is not safe to call badness() even if this task is
	  ->group_leader, has_intersects_mems_allowed() assumes
	  it is safe to iterate over ->thread_group list.

	- even worse, badness() can hit ->signal == NULL

Add the pid_alive() check to ensure __unhash_process() was not called.

Also, use "task" instead of task->group_leader. badness() should return
the same result for any sub-thread. Currently this is not true, but
this should be changed anyway.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 2 deletions

diff --git a/fs/proc/base.c b/fs/proc/base.c
index a7310841c831..b1f6e62773d3 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -442,12 +442,13 @@ static const struct file_operations proc_lstats_operations = {
 unsigned long badness(struct task_struct *p, unsigned long uptime);
 static int proc_oom_score(struct task_struct *task, char *buffer)
 {
-        unsigned long points;
+        unsigned long points = 0;
         struct timespec uptime;
 
         do_posix_clock_monotonic_gettime(&uptime);
         read_lock(&tasklist_lock);
-        points = badness(task->group_leader, uptime.tv_sec);
+        if (pid_alive(task))
+                points = badness(task, uptime.tv_sec);
         read_unlock(&tasklist_lock);
         return sprintf(buffer, "%lu\n", points);
 }