In
commit cdd1cf799bd24ac0a4184549601ae302267301c5 Author: Chris Wilson chris@chris-wilson.co.uk 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 thellstrom@vmware.com Signed-off-by: Chris Wilson chris@chris-wilson.co.uk Cc: Thomas Hellstrom thellstrom@vmware.com Cc: Takashi Iwai tiwai@suse.de Cc: Daniel Vetter daniel.vetter@ffwll.ch --- 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; }