From 83eb64c85b80959549c114365016276f318afeb2 Mon Sep 17 00:00:00 2001 From: Chris Wilson Date: Wed, 25 Nov 2015 14:39:02 +0000 Subject: [PATCH] drm: Drop dev->event_lock spinlock around faulting copy_to_user() In commit cdd1cf799bd24ac0a4184549601ae302267301c5 Author: Chris Wilson Date: Thu Dec 4 21:03:25 2014 +0000 drm: Make drm_read() more robust against multithreaded races I fixed the races by serialising the use of the event by extending the dev->event_lock. However, as Thomas pointed out, the copy_to_user() may fault (even the __copy_to_user_inatomic() variant used here) and calling into the driver backend with the spinlock held is bad news. Therefore we have to drop the spinlock before the copy, but that exposes us to the old race whereby a second reader could see an out-of-order event (as the first reader may claim the first request but fail to copy it back to userspace and so on returning it to the event list it will be behind the current event being copied by the second reader). Reported-by: Thomas Hellstrom Signed-off-by: Chris Wilson Cc: Thomas Hellstrom Cc: Takashi Iwai Cc: Daniel Vetter Link: http://patchwork.freedesktop.org/patch/msgid/1448462343-2072-1-git-send-email-chris@chris-wilson.co.uk Reviewed-by: Thomas Hellstrom Signed-off-by: Daniel Vetter --- drivers/gpu/drm/drm_fops.c | 40 +++++++++++++++++++++++--------------- 1 file changed, 24 insertions(+), 16 deletions(-) diff --git a/drivers/gpu/drm/drm_fops.c b/drivers/gpu/drm/drm_fops.c index c59ce4d0ef75..eb8702d39e7d 100644 --- a/drivers/gpu/drm/drm_fops.c +++ b/drivers/gpu/drm/drm_fops.c @@ -488,9 +488,19 @@ ssize_t drm_read(struct file *filp, char __user *buffer, if (!access_ok(VERIFY_WRITE, buffer, count)) return -EFAULT; - spin_lock_irq(&dev->event_lock); for (;;) { - if (list_empty(&file_priv->event_list)) { + struct drm_pending_event *e = NULL; + + spin_lock_irq(&dev->event_lock); + if (!list_empty(&file_priv->event_list)) { + e = list_first_entry(&file_priv->event_list, + struct drm_pending_event, link); + file_priv->event_space += e->event->length; + list_del(&e->link); + } + spin_unlock_irq(&dev->event_lock); + + if (e == NULL) { if (ret) break; @@ -499,36 +509,34 @@ ssize_t drm_read(struct file *filp, char __user *buffer, break; } - spin_unlock_irq(&dev->event_lock); ret = wait_event_interruptible(file_priv->event_wait, !list_empty(&file_priv->event_list)); - spin_lock_irq(&dev->event_lock); if (ret < 0) break; ret = 0; } else { - struct drm_pending_event *e; - - e = list_first_entry(&file_priv->event_list, - struct drm_pending_event, link); - if (e->event->length + ret > count) + unsigned length = e->event->length; + + if (length > count - ret) { +put_back_event: + spin_lock_irq(&dev->event_lock); + file_priv->event_space -= length; + list_add(&e->link, &file_priv->event_list); + spin_unlock_irq(&dev->event_lock); break; + } - if (__copy_to_user_inatomic(buffer + ret, - e->event, e->event->length)) { + if (copy_to_user(buffer + ret, e->event, length)) { if (ret == 0) ret = -EFAULT; - break; + goto put_back_event; } - file_priv->event_space += e->event->length; - ret += e->event->length; - list_del(&e->link); + ret += length; e->destroy(e); } } - spin_unlock_irq(&dev->event_lock); return ret; } -- 2.30.2