kthread: make kthread_create() killable

Any user process callers of wait_for_completion() except global init
process might be chosen by the OOM killer while waiting for completion()
call by some other process which does memory allocation.  See
CVE-2012-4398 "kernel: request_module() OOM local DoS" can happen.

When such users are chosen by the OOM killer when they are waiting for
completion() in TASK_UNINTERRUPTIBLE, the system will be kept stressed
due to memory starvation because the OOM killer cannot kill such users.

kthread_create() is one of such users and this patch fixes the problem
for kthreadd by making kthread_create() killable - the same approach
used for fixing CVE-2012-4398.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Oleg Nesterov <oleg@redhat.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 55 insertions, 18 deletions

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 760e86df8c20..b5ae3ee860a9 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -33,7 +33,7 @@ struct kthread_create_info
 
         /* Result passed back to kthread_create() from kthreadd. */
         struct task_struct *result;
-        struct completion done;
+        struct completion *done;
 
         struct list_head list;
 };
@@ -178,6 +178,7 @@ static int kthread(void *_create)
         struct kthread_create_info *create = _create;
         int (*threadfn)(void *data) = create->threadfn;
         void *data = create->data;
+        struct completion *done;
         struct kthread self;
         int ret;
 
@@ -187,10 +188,16 @@ static int kthread(void *_create)
         init_completion(&self.parked);
         current->vfork_done = &self.exited;
 
+        /* If user was SIGKILLed, I release the structure. */
+        done = xchg(&create->done, NULL);
+        if (!done) {
+                kfree(create);
+                do_exit(-EINTR);
+        }
         /* OK, tell user we're spawned, wait for stop or wakeup */
         __set_current_state(TASK_UNINTERRUPTIBLE);
         create->result = current;
-        complete(&create->done);
+        complete(done);
         schedule();
 
         ret = -EINTR;
@@ -223,8 +230,15 @@ static void create_kthread(struct kthread_create_info *create)
         /* We want our own signal handler (we take no signals by default). */
         pid = kernel_thread(kthread, create, CLONE_FS | CLONE_FILES | SIGCHLD);
         if (pid < 0) {
+                /* If user was SIGKILLed, I release the structure. */
+                struct completion *done = xchg(&create->done, NULL);
+
+                if (!done) {
+                        kfree(create);
+                        return;
+                }
                 create->result = ERR_PTR(pid);
-                complete(&create->done);
+                complete(done);
         }
 }
 
@@ -255,36 +269,59 @@ struct task_struct *kthread_create_on_node(int (*threadfn)(void *data),
                                            const char namefmt[],
                                            ...)
 {
-        struct kthread_create_info create;
-
-        create.threadfn = threadfn;
-        create.data = data;
-        create.node = node;
-        init_completion(&create.done);
+        DECLARE_COMPLETION_ONSTACK(done);
+        struct task_struct *task;
+        struct kthread_create_info *create = kmalloc(sizeof(*create),
+                                                     GFP_KERNEL);
+
+        if (!create)
+                return ERR_PTR(-ENOMEM);
+        create->threadfn = threadfn;
+        create->data = data;
+        create->node = node;
+        create->done = &done;
 
         spin_lock(&kthread_create_lock);
-        list_add_tail(&create.list, &kthread_create_list);
+        list_add_tail(&create->list, &kthread_create_list);
         spin_unlock(&kthread_create_lock);
 
         wake_up_process(kthreadd_task);
-        wait_for_completion(&create.done);
-
-        if (!IS_ERR(create.result)) {
+        /*
+         * Wait for completion in killable state, for I might be chosen by
+         * the OOM killer while kthreadd is trying to allocate memory for
+         * new kernel thread.
+         */
+        if (unlikely(wait_for_completion_killable(&done))) {
+                /*
+                 * If I was SIGKILLed before kthreadd (or new kernel thread)
+                 * calls complete(), leave the cleanup of this structure to
+                 * that thread.
+                 */
+                if (xchg(&create->done, NULL))
+                        return ERR_PTR(-ENOMEM);
+                /*
+                 * kthreadd (or new kernel thread) will call complete()
+                 * shortly.
+                 */
+                wait_for_completion(&done);
+        }
+        task = create->result;
+        if (!IS_ERR(task)) {
                 static const struct sched_param param = { .sched_priority = 0 };
                 va_list args;
 
                 va_start(args, namefmt);
-                vsnprintf(create.result->comm, sizeof(create.result->comm),
-                          namefmt, args);
+                vsnprintf(task->comm, sizeof(task->comm), namefmt, args);
                 va_end(args);
                 /*
                  * root may have changed our (kthreadd's) priority or CPU mask.
                  * The kernel thread should not inherit these properties.
                  */
-                sched_setscheduler_nocheck(create.result, SCHED_NORMAL, &param);
-                set_cpus_allowed_ptr(create.result, cpu_all_mask);
+                sched_setscheduler_nocheck(task, SCHED_NORMAL, &param);
+                set_cpus_allowed_ptr(task, cpu_all_mask);
         }
-        return create.result;
+        kfree(create);
+        return task;
 }
 EXPORT_SYMBOL(kthread_create_on_node);