[PATCH] gpio: sloppy-logic-analyzer: fix use-after-free via debugfs trigger on unbind

[PATCH] gpio: sloppy-logic-analyzer: fix use-after-free via debugfs trigger on unbind

[Cengiz Can]

The "trigger" debugfs file has a hand-rolled ->write handler
(trigger_write()) that dereferences the per-device gpio_la_poll_priv. The
file is created with debugfs_create_file_unsafe(), and the handler never
takes a debugfs reference. Nothing keeps the object alive while the
handler runs.

The sibling "buf_size" and "capture" files are safe: they use
DEFINE_DEBUGFS_ATTRIBUTE(), whose accessors take a debugfs reference. Only
"trigger" omits it.

priv is allocated with devm_kzalloc(). devres frees it when the platform
device is unbound. debugfs_create_file_unsafe() installs no full_proxy
wrapper, so debugfs_remove_recursive() in gpio_la_poll_remove() does not
wait for an in-flight trigger_write(). The blob_lock taken there does not
help, because trigger_write() never takes it. A write that races an unbind
therefore writes into freed memory:

  trigger_write()                  gpio_la_poll_remove()
    priv = m->private
    buf = memdup_user()  [may sleep]
                                     mutex_lock(&priv->blob_lock)
                                     debugfs_remove_recursive()  [no wait]
                                     mutex_unlock(&priv->blob_lock)
                                   (remove returns; devres frees priv)
    priv->trig_data = buf   <-- use-after-free write
    priv->trig_len  = count

The race is reachable by root via
/sys/bus/platform/drivers/gpio-sloppy-logic-analyzer/unbind.

Create "trigger" with debugfs_create_file() instead. Its full_proxy
wrapper makes debugfs_remove_recursive() drain any in-flight ->write
before it returns. "buf_size" and "capture" are already safe and are left
unchanged.

The mechanism is confirmed under KASAN with a minimal reproducer of the
same debugfs_create_file_unsafe() plus devm_kzalloc() pattern (available
on request); it produces a slab-use-after-free write in the handler.

Fixes: 7828b7bbbf20 ("gpio: add sloppy logic analyzer using polling")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4.8
Signed-off-by: Cengiz Can <cengiz.can@canonical.com>
---
 drivers/gpio/gpio-sloppy-logic-analyzer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpio/gpio-sloppy-logic-analyzer.c b/drivers/gpio/gpio-sloppy-logic-analyzer.c
index 2bbd308ca08e..c356cf2ed688 100644
--- a/drivers/gpio/gpio-sloppy-logic-analyzer.c
+++ b/drivers/gpio/gpio-sloppy-logic-analyzer.c
@@ -290,7 +290,7 @@ static int gpio_la_poll_probe(struct platform_device *pdev)
 	debugfs_create_ulong("delay_ns_acquisition", 0400, priv->debug_dir, &priv->acq_delay);
 	debugfs_create_file_unsafe("buf_size", 0600, priv->debug_dir, priv, &fops_buf_size);
 	debugfs_create_file_unsafe("capture", 0200, priv->debug_dir, priv, &fops_capture);
-	debugfs_create_file_unsafe("trigger", 0200, priv->debug_dir, priv, &fops_trigger);
+	debugfs_create_file("trigger", 0200, priv->debug_dir, priv, &fops_trigger);
 
 	return 0;
 }
-- 
2.43.0

Re: [PATCH] gpio: sloppy-logic-analyzer: fix use-after-free via debugfs trigger on unbind

[Wolfram Sang]

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>  	debugfs_create_file_unsafe("buf_size", 0600, priv->debug_dir, priv, &fops_buf_size);
>  	debugfs_create_file_unsafe("capture", 0200, priv->debug_dir, priv, &fops_capture);
> -	debugfs_create_file_unsafe("trigger", 0200, priv->debug_dir, priv, &fops_trigger);
> +	debugfs_create_file("trigger", 0200, priv->debug_dir, priv, &fops_trigger);

What about removing the other _unsafe calls as well? TBH, I can't recall
why I used them. Doesn't make sense from today's perspective.


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