udmabuf: fix memory leak on last export_udmabuf() error path

In export_udmabuf(), if dma_buf_fd() fails because the FD table is full, a
dma_buf owning the udmabuf has already been created; but the error handling
in udmabuf_create() will tear down the udmabuf without doing anything about
the containing dma_buf.

This leaves a dma_buf in memory that contains a dangling pointer; though
that doesn't seem to lead to anything bad except a memory leak.

Fix it by moving the dma_buf_fd() call out of export_udmabuf() so that we
can give it different error handling.

Note that the shape of this code changed a lot in commit 5e72b2b4
("udmabuf: convert udmabuf driver to use folios"); but the memory leak
seems to have existed since the introduction of udmabuf.

Fixes: fbb0de79 ("Add udmabuf misc device")
Acked-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241204-udmabuf-fixes-v2-3-23887289de1c@google.com

drivers/dma-buf/udmabuf.c

@@ -317,12 +317,10 @@ static int check_memfd_seals(struct file *memfd)
 	return 0;
 }
 
-static int export_udmabuf(struct udmabuf *ubuf,
-			  struct miscdevice *device,
-			  u32 flags)
+static struct dma_buf *export_udmabuf(struct udmabuf *ubuf,
+				      struct miscdevice *device)
 {
 	DEFINE_DMA_BUF_EXPORT_INFO(exp_info);
-	struct dma_buf *buf;
 
 	ubuf->device = device;
 	exp_info.ops  = &udmabuf_ops;
@@ -330,11 +328,7 @@ static int export_udmabuf(struct udmabuf *ubuf,
 	exp_info.priv = ubuf;
 	exp_info.flags = O_RDWR;
 
-	buf = dma_buf_export(&exp_info);
-	if (IS_ERR(buf))
-		return PTR_ERR(buf);
-
-	return dma_buf_fd(buf, flags);
+	return dma_buf_export(&exp_info);
 }
 
 static long udmabuf_pin_folios(struct udmabuf *ubuf, struct file *memfd,
@@ -391,6 +385,7 @@ static long udmabuf_create(struct miscdevice *device,
 	struct folio **folios = NULL;
 	pgoff_t pgcnt = 0, pglimit;
 	struct udmabuf *ubuf;
+	struct dma_buf *dmabuf;
 	long ret = -EINVAL;
 	u32 i, flags;
 
@@ -455,9 +450,20 @@ out_unlock:
 	}
 
 	flags = head->flags & UDMABUF_FLAGS_CLOEXEC ? O_CLOEXEC : 0;
-	ret = export_udmabuf(ubuf, device, flags);
-	if (ret < 0)
+	dmabuf = export_udmabuf(ubuf, device);
+	if (IS_ERR(dmabuf)) {
+		ret = PTR_ERR(dmabuf);
 		goto err;
+	}
+	/*
+	 * Ownership of ubuf is held by the dmabuf from here.
+	 * If the following dma_buf_fd() fails, dma_buf_put() cleans up both the
+	 * dmabuf and the ubuf (through udmabuf_ops.release).
+	 */
+
+	ret = dma_buf_fd(dmabuf, flags);
+	if (ret < 0)
+		dma_buf_put(dmabuf);
 
 	kvfree(folios);
 	return ret;