4.13-stable review patch. If anyone has any objections, please let me know.
------------------ From: Peter Zijlstra <[email protected]> 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 <[email protected]> Signed-off-by: Peter Zijlstra (Intel) <[email protected]> Reviewed-by: Thomas Gleixner <[email protected]> Cc: Gratian Crisan <[email protected]> Cc: Linus Torvalds <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: [email protected] Cc: syzbot <bot+2af19c9e1ffe4d4ee1d16c56ae7580feaee75...@syzkaller.appspotmail.com> Cc: [email protected] Fixes: c74aef2d06a9 ("futex: Fix pi_state->owner serialization") Link: http://lkml.kernel.org/r/[email protected] Signed-off-by: Ingo Molnar <[email protected]> Signed-off-by: Greg Kroah-Hartman <[email protected]> --- kernel/futex.c | 23 ++++++++++++++++++++--- 1 file changed, 20 insertions(+), 3 deletions(-) --- a/kernel/futex.c +++ b/kernel/futex.c @@ -901,11 +901,27 @@ void exit_pi_state_list(struct task_stru */ 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); @@ -916,8 +932,10 @@ void exit_pi_state_list(struct task_stru * 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; } @@ -925,9 +943,8 @@ void exit_pi_state_list(struct task_stru 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);
