3.0-stable review patch. If anyone has any objections, please let me know.
------------------ From: Oleg Nesterov <[email protected]> commit 971316f0503a5c50633d07b83b6db2f15a3a5b00 upstream. signalfd_cleanup() ensures that ->signalfd_wqh is not used, but this is not enough. eppoll_entry->whead still points to the memory we are going to free, ep_unregister_pollwait()->remove_wait_queue() is obviously unsafe. Change ep_poll_callback(POLLFREE) to set eppoll_entry->whead = NULL, change ep_unregister_pollwait() to check pwq->whead != NULL under rcu_read_lock() before remove_wait_queue(). We add the new helper, ep_remove_wait_queue(), for this. This works because sighand_cachep is SLAB_DESTROY_BY_RCU and because ->signalfd_wqh is initialized in sighand_ctor(), not in copy_sighand. ep_unregister_pollwait()->remove_wait_queue() can play with already freed and potentially reused ->sighand, but this is fine. This memory must have the valid ->signalfd_wqh until rcu_read_unlock(). Reported-by: Maxime Bizon <[email protected]> Signed-off-by: Oleg Nesterov <[email protected]> Signed-off-by: Linus Torvalds <[email protected]> Signed-off-by: Greg Kroah-Hartman <[email protected]> --- fs/eventpoll.c | 30 +++++++++++++++++++++++++++--- fs/signalfd.c | 6 +++++- 2 files changed, 32 insertions(+), 4 deletions(-) --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -299,6 +299,11 @@ static inline int ep_is_linked(struct li return !list_empty(p); } +static inline struct eppoll_entry *ep_pwq_from_wait(wait_queue_t *p) +{ + return container_of(p, struct eppoll_entry, wait); +} + /* Get the "struct epitem" from a wait queue pointer */ static inline struct epitem *ep_item_from_wait(wait_queue_t *p) { @@ -446,6 +451,18 @@ static void ep_poll_safewake(wait_queue_ put_cpu(); } +static void ep_remove_wait_queue(struct eppoll_entry *pwq) +{ + wait_queue_head_t *whead; + + rcu_read_lock(); + /* If it is cleared by POLLFREE, it should be rcu-safe */ + whead = rcu_dereference(pwq->whead); + if (whead) + remove_wait_queue(whead, &pwq->wait); + rcu_read_unlock(); +} + /* * This function unregisters poll callbacks from the associated file * descriptor. Must be called with "mtx" held (or "epmutex" if called from @@ -460,7 +477,7 @@ static void ep_unregister_pollwait(struc pwq = list_first_entry(lsthead, struct eppoll_entry, llink); list_del(&pwq->llink); - remove_wait_queue(pwq->whead, &pwq->wait); + ep_remove_wait_queue(pwq); kmem_cache_free(pwq_cache, pwq); } } @@ -827,9 +844,16 @@ static int ep_poll_callback(wait_queue_t struct epitem *epi = ep_item_from_wait(wait); struct eventpoll *ep = epi->ep; - /* the caller holds eppoll_entry->whead->lock */ - if ((unsigned long)key & POLLFREE) + if ((unsigned long)key & POLLFREE) { + ep_pwq_from_wait(wait)->whead = NULL; + /* + * whead = NULL above can race with ep_remove_wait_queue() + * which can do another remove_wait_queue() after us, so we + * can't use __remove_wait_queue(). whead->lock is held by + * the caller. + */ list_del_init(&wait->task_list); + } spin_lock_irqsave(&ep->lock, flags); --- a/fs/signalfd.c +++ b/fs/signalfd.c @@ -33,7 +33,11 @@ void signalfd_cleanup(struct sighand_struct *sighand) { wait_queue_head_t *wqh = &sighand->signalfd_wqh; - + /* + * The lockless check can race with remove_wait_queue() in progress, + * but in this case its caller should run under rcu_read_lock() and + * sighand_cachep is SLAB_DESTROY_BY_RCU, we can safely return. + */ if (likely(!waitqueue_active(wqh))) return; -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
