futex: Fix more put_pi_state() vs. exit_pi_state_list() races

Dmitry (through syzbot) reported being able to trigger the WARN in
get_pi_state() and a use-after-free on:

	raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);

Both are due to this race:

  exit_pi_state_list()				put_pi_state()

  lock(&curr->pi_lock)
  while() {
	pi_state = list_first_entry(head);
	hb = hash_futex(&pi_state->key);
	unlock(&curr->pi_lock);

						dec_and_test(&pi_state->refcount);

	lock(&hb->lock)
	lock(&pi_state->pi_mutex.wait_lock)	// uaf if pi_state free'd
	lock(&curr->pi_lock);

	....

	unlock(&curr->pi_lock);
	get_pi_state();				// WARN; refcount==0

The problem is we take the reference count too late, and don't allow it
being 0. Fix it by using inc_not_zero() and simply retrying the loop
when we fail to get a refcount. In that case put_pi_state() should
remove the entry from the list.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Gratian Crisan <gratian.crisan@ni.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: dvhart@infradead.org
Cc: syzbot <bot+2af19c9e1ffe4d4ee1d16c56ae7580feaee75765@syzkaller.appspotmail.com>
Cc: syzkaller-bugs@googlegroups.com
Cc: <stable@vger.kernel.org>
Fixes: c74aef2d06a9 ("futex: Fix pi_state->owner serialization")
Link: http://lkml.kernel.org/r/20171031101853.xpfh72y643kdfhjs@hirez.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 20 insertions, 3 deletions

diff --git a/kernel/futex.c b/kernel/futex.c
index 0518a0bfc746..ca5bb9cba5cf 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -903,11 +903,27 @@ void exit_pi_state_list(struct task_struct *curr)
          */
         raw_spin_lock_irq(&curr->pi_lock);
         while (!list_empty(head)) {
-
                 next = head->next;
                 pi_state = list_entry(next, struct futex_pi_state, list);
                 key = pi_state->key;
                 hb = hash_futex(&key);
+
+                /*
+                 * We can race against put_pi_state() removing itself from the
+                 * list (a waiter going away). put_pi_state() will first
+                 * decrement the reference count and then modify the list, so
+                 * its possible to see the list entry but fail this reference
+                 * acquire.
+                 *
+                 * In that case; drop the locks to let put_pi_state() make
+                 * progress and retry the loop.
+                 */
+                if (!atomic_inc_not_zero(&pi_state->refcount)) {
+                        raw_spin_unlock_irq(&curr->pi_lock);
+                        cpu_relax();
+                        raw_spin_lock_irq(&curr->pi_lock);
+                        continue;
+                }
                 raw_spin_unlock_irq(&curr->pi_lock);
 
                 spin_lock(&hb->lock);
@@ -918,8 +934,10 @@ void exit_pi_state_list(struct task_struct *curr)
                  * task still owns the PI-state:
                  */
                 if (head->next != next) {
+                        /* retain curr->pi_lock for the loop invariant */
                         raw_spin_unlock(&pi_state->pi_mutex.wait_lock);
                         spin_unlock(&hb->lock);
+                        put_pi_state(pi_state);
                         continue;
                 }
 
@@ -927,9 +945,8 @@ void exit_pi_state_list(struct task_struct *curr)
                 WARN_ON(list_empty(&pi_state->list));
                 list_del_init(&pi_state->list);
                 pi_state->owner = NULL;
-                raw_spin_unlock(&curr->pi_lock);
 
-                get_pi_state(pi_state);
+                raw_spin_unlock(&curr->pi_lock);
                 raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
                 spin_unlock(&hb->lock);