[tip:core/urgent] futex: Fix more put_pi_state() vs. exit_pi_state_list() races

tip-bot for Peter Zijlstra
2017-10-31T12:12:03Z

Commit-ID: d5f6ac33189af48a0dc011190af5144947a30a76
Gitweb: https://git.kernel.org/tip/d5f6ac33189af48a0dc011190af5144947a30a76
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Tue, 31 Oct 2017 11:18:53 +0100
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 31 Oct 2017 11:54:52 +0100

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>
Cc: Gratian Crisan <gratian.crisan@ni.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dvhart@infradead.org
Cc: syzbot <bot+2af19c9e1ffe4d4ee1d16c56ae7580feaee75765@syzkaller.appspotmail.com>
Cc: syzkaller-bugs@googlegroups.com
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>
---
kernel/futex.c | 23 ++++++++++++++++++++---
1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 0518a0b..ca5bb9c 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);