This is a note to let you know that I've just added the patch titled
futex: Fix more put_pi_state() vs. exit_pi_state_list() races
to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
futex-fix-more-put_pi_state-vs.-exit_pi_state_list-races.patch
and it can be found in the queue-4.9 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From foo@baz Thu Mar 4 02:09:29 PM CET 2021
From: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
Date: Mon, 1 Mar 2021 18:31:55 +0100
Subject: futex: Fix more put_pi_state() vs. exit_pi_state_list() races
To: stable@xxxxxxxxxxxxxxx
Cc: Lee Jones <lee.jones@xxxxxxxxxx>, "Luis Claudio R. Goncalves" <lgoncalv@xxxxxxxxxx>
Message-ID: <YD0lC+fuTMO3BKSw@xxxxxxxxxxxxxxx>
Content-Disposition: inline
From: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
commit 153fbd1226fb30b8630802aa5047b8af5ef53c9f upstream.
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@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Gratian Crisan <gratian.crisan@xxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: dvhart@xxxxxxxxxxxxx
Cc: syzbot <bot+2af19c9e1ffe4d4ee1d16c56ae7580feaee75765@xxxxxxxxxxxxxxxxxxxxxxxxx>
Cc: syzkaller-bugs@xxxxxxxxxxxxxxxx
Cc: <stable@xxxxxxxxxxxxxxx>
Fixes: c74aef2d06a9 ("futex: Fix pi_state->owner serialization")
Link: http://lkml.kernel.org/r/20171031101853.xpfh72y643kdfhjs@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/futex.c | 23 ++++++++++++++++++++---
1 file changed, 20 insertions(+), 3 deletions(-)
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -941,11 +941,27 @@ static void exit_pi_state_list(struct ta
*/
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);
@@ -956,8 +972,10 @@ static void exit_pi_state_list(struct ta
* 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;
}
@@ -965,9 +983,8 @@ static void exit_pi_state_list(struct ta
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);
Patches currently in stable-queue which might be from ben@xxxxxxxxxxxxxxx are
queue-4.9/futex-cleanup-refcounting.patch
queue-4.9/futex-fix-more-put_pi_state-vs.-exit_pi_state_list-races.patch
queue-4.9/futex-futex_unlock_pi-determinism.patch
queue-4.9/futex-cleanup-variable-names-for-futex_top_waiter.patch
queue-4.9/futex-don-t-enable-irqs-unconditionally-in-put_pi_state.patch
queue-4.9/futex-fix-pi_state-owner-serialization.patch
queue-4.9/futex-pull-rt_mutex_futex_unlock-out-from-under-hb-lock.patch
