diff mbox series

[v7,7/8] blktrace: ensure our debugfs dir exists

Message ID 20200619204730.26124-8-mcgrof@kernel.org
State New
Headers show
Series blktrace: fix debugfs use after free | expand

Commit Message

Luis Chamberlain June 19, 2020, 8:47 p.m. UTC
We make an assumption that a debugfs directory exists, but since
this can fail ensure it exists before allowing blktrace setup to
complete. Otherwise we end up stuffing blktrace files on the debugfs
root directory. In the worst case scenario this *in theory* can create
an eventual panic *iff* in the future a similarly named file is created
prior on the debugfs root directory. This theoretical crash can happen
due to a recursive removal followed by a specific dentry removal.

This doesn't fix any known crash, however I have seen the files
go into the main debugfs root directory in cases where the debugfs
directory was not created due to other internal bugs with blktrace
now fixed.

blktrace is also completely useless without this directory, so
this ensures to userspace we only setup blktrace if the kernel
can stuff files where they are supposed to go into.

debugfs directory creations typically aren't checked for, and we have
maintainers doing sweep removals of these checks, but since we need this
check to ensure proper userspace blktrace functionality we make sure
to annotate the justification for the check.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
---
 kernel/trace/blktrace.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

Comments

Christoph Hellwig June 20, 2020, 7:29 a.m. UTC | #1
Looks good,

Reviewed-by: Christoph Hellwig <hch@lst.de>
Bart Van Assche June 20, 2020, 5:33 p.m. UTC | #2
On 2020-06-19 13:47, Luis Chamberlain wrote:
> diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
> index e6e2d25fdbd6..098780ec018f 100644
> --- a/kernel/trace/blktrace.c
> +++ b/kernel/trace/blktrace.c
> @@ -538,6 +538,18 @@ static int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
>  #endif
>  		bt->dir = dir = debugfs_create_dir(buts->name, blk_debugfs_root);
>  
> +	/*
> +	 * As blktrace relies on debugfs for its interface the debugfs directory
> +	 * is required, contrary to the usual mantra of not checking for debugfs
> +	 * files or directories.
> +	 */
> +	if (IS_ERR_OR_NULL(dir)) {
> +		pr_warn("debugfs_dir not present for %s so skipping\n",
> +			buts->name);

Maybe mention in the message what is being skipped (block tracing)?

> +		ret = -ENOENT;
> +		goto err;
> +	}
> +
>  	bt->dev = dev;
>  	atomic_set(&bt->dropped, 0);
>  	INIT_LIST_HEAD(&bt->running_list);

Anyway:

Reviewed-by: Bart Van Assche <bvanassche@acm.org>
diff mbox series

Patch

diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index e6e2d25fdbd6..098780ec018f 100644
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -538,6 +538,18 @@  static int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
 #endif
 		bt->dir = dir = debugfs_create_dir(buts->name, blk_debugfs_root);
 
+	/*
+	 * As blktrace relies on debugfs for its interface the debugfs directory
+	 * is required, contrary to the usual mantra of not checking for debugfs
+	 * files or directories.
+	 */
+	if (IS_ERR_OR_NULL(dir)) {
+		pr_warn("debugfs_dir not present for %s so skipping\n",
+			buts->name);
+		ret = -ENOENT;
+		goto err;
+	}
+
 	bt->dev = dev;
 	atomic_set(&bt->dropped, 0);
 	INIT_LIST_HEAD(&bt->running_list);