Re: [PATCH v6 6/6] blktrace: fix debugfs use after free
From: Luis Chamberlain <mcgrof@kernel.org>
Date: 2020-06-10 21:52:23
Also in:
linux-fsdevel, linux-mm, lkml
Subsystem:
block layer, the rest, tracing · Maintainers:
Jens Axboe, Linus Torvalds, Steven Rostedt, Masami Hiramatsu
So, upon updating the commit log, and moving the empty directory check into another patch, I realized we might be able to simplify this now even further still. Patch below. The key of the issue was that the use after free happens when a recursive removal happens, and then later a specific dentry removal happens. This happened for make_request block drivers when using the whole disk, but since we *don't* have any other users of the directory for the others cases, this in theory shuld not happen for them either. I'll try to shoot some bullets at this.
diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index 7ff2ea5cd05e..5cea04c05e09 100644
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c@@ -524,10 +524,16 @@ static int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev, if (!bt->msg_data) goto err; - ret = -ENOENT; - - dir = debugfs_lookup(buts->name, blk_debugfs_root); - if (!dir) + /* + * When tracing whole make_request drivers (multiqueue) block devices, + * reuse the existing debugfs directory created by the block layer on + * init. For request-based block devices, all partitions block devices, + * and scsi-generic block devices we create a temporary new debugfs + * directory that will be removed once the trace ends. + */ + if (queue_is_mq(q)) + dir = q->debugfs_dir; + else bt->dir = dir = debugfs_create_dir(buts->name, blk_debugfs_root); bt->dev = dev;
@@ -565,8 +571,6 @@ static int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev, ret = 0; err: - if (dir && !bt->dir) - dput(dir); if (ret) blk_trace_free(bt); return ret;