dma-buf/poll: Get a file reference for outstanding fence callbacks

This makes sure we don't hit the

	BUG_ON(dmabuf->cb_in.active || dmabuf->cb_out.active);

in dma_buf_release, which could be triggered by user space closing the
dma-buf file description while there are outstanding fence callbacks
from dma_buf_poll.

Cc: stable@vger.kernel.org
Signed-off-by: Michel Dänzer <mdaenzer@redhat.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210723075857.4065-1-michel@daenzer.net
Signed-off-by: Christian König <christian.koenig@amd.com>

Author: Michel Dänzer

drivers/dma-buf/dma-buf.c

@@ -67,12 +67,9 @@ static void dma_buf_release(struct dentry *dentry)
 	BUG_ON(dmabuf->vmapping_counter);
 
 	/*
-	 * Any fences that a dma-buf poll can wait on should be signaled
-	 * before releasing dma-buf. This is the responsibility of each
-	 * driver that uses the reservation objects.
-	 *
-	 * If you hit this BUG() it means someone dropped their ref to the
-	 * dma-buf while still having pending operation to the buffer.
+	 * If you hit this BUG() it could mean:
+	 * * There's a file reference imbalance in dma_buf_poll / dma_buf_poll_cb or somewhere else
+	 * * dmabuf->cb_in/out.active are non-0 despite no pending fence callback
 	 */
 	BUG_ON(dmabuf->cb_in.active || dmabuf->cb_out.active);
 
@@ -200,13 +197,16 @@ static loff_t dma_buf_llseek(struct file *file, loff_t offset, int whence)
 static void dma_buf_poll_cb(struct dma_fence *fence, struct dma_fence_cb *cb)
 {
 	struct dma_buf_poll_cb_t *dcb = (struct dma_buf_poll_cb_t *)cb;
+	struct dma_buf *dmabuf = container_of(dcb->poll, struct dma_buf, poll);
 	unsigned long flags;
 
 	spin_lock_irqsave(&dcb->poll->lock, flags);
 	wake_up_locked_poll(dcb->poll, dcb->active);
 	dcb->active = 0;
 	spin_unlock_irqrestore(&dcb->poll->lock, flags);
 	dma_fence_put(fence);
+	/* Paired with get_file in dma_buf_poll */
+	fput(dmabuf->file);
 }
 
 static bool dma_buf_poll_add_cb(struct dma_resv *resv, bool write,
@@ -259,6 +259,9 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll)
 		spin_unlock_irq(&dmabuf->poll.lock);
 
 		if (events & EPOLLOUT) {
+			/* Paired with fput in dma_buf_poll_cb */
+			get_file(dmabuf->file);
+
 			if (!dma_buf_poll_add_cb(resv, true, dcb))
 				/* No callback queued, wake up any other waiters */
 				dma_buf_poll_cb(NULL, &dcb->cb);
@@ -279,6 +282,9 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll)
 		spin_unlock_irq(&dmabuf->poll.lock);
 
 		if (events & EPOLLIN) {
+			/* Paired with fput in dma_buf_poll_cb */
+			get_file(dmabuf->file);
+
 			if (!dma_buf_poll_add_cb(resv, false, dcb))
 				/* No callback queued, wake up any other waiters */
 				dma_buf_poll_cb(NULL, &dcb->cb);