Re: [CFT][PATCH] fix descriptor uses in sound/core/compress_offload.c

[Al Viro <viro@zeniv.linux.org.uk>]

On Thu, Dec 26, 2024 at 09:31:22PM +0000, Al Viro wrote:
> On Thu, Dec 26, 2024 at 08:00:18PM +0100, Jaroslav Kysela wrote:
> 
> >   I already made almost similar patch:
> > 
> > https://lore.kernel.org/linux-sound/20241217100726.732863-1-perex@perex.cz/
> 
> Umm...  The only problem with your variant is that dma_buf_get()
> is wrong here - it should be get_dma_buf() on actual objects,
> and it should be done before fd_install().

Incremental on top of what just got merged into mainline:

Grab the references to dmabuf before moving them into descriptor
table - trying to do that by descriptor afterwards might end up getting
a different object, with a dangling reference left in task->{input,output}

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
diff --git a/sound/core/compress_offload.c b/sound/core/compress_offload.c
index edf5aadf38e5..543c7f525f84 100644
--- a/sound/core/compress_offload.c
+++ b/sound/core/compress_offload.c
@@ -1053,13 +1053,13 @@ static int snd_compr_task_new(struct snd_compr_stream *stream, struct snd_compr_
 		put_unused_fd(fd_i);
 		goto cleanup;
 	}
+	/* keep dmabuf reference until freed with task free ioctl */
+	get_dma_buf(task->input);
+	get_dma_buf(task->output);
 	fd_install(fd_i, task->input->file);
 	fd_install(fd_o, task->output->file);
 	utask->input_fd = fd_i;
 	utask->output_fd = fd_o;
-	/* keep dmabuf reference until freed with task free ioctl */
-	dma_buf_get(utask->input_fd);
-	dma_buf_get(utask->output_fd);
 	list_add_tail(&task->list, &stream->runtime->tasks);
 	stream->runtime->total_tasks++;
 	return 0;