[PATCH] usb: gadget: u_audio: Fix use-after-free on sound card disconnect

[Sonali Pradhan <sonalipradhan@google.com>]

g_audio_cleanup() invokes snd_card_free_when_closed() to initiate sound
card teardown and immediately frees the underlying struct snd_uac_chip
context. However, snd_card_free_when_closed() returns asynchronously
while ALSA control elements (kctls) remain open in userspace.

When userspace control applications access or close these open file
descriptors, kctl callbacks attempt to dereference kctl->private_data
pointing to &uac->c_prm or &uac->p_prm within the freed uac structure,
resulting in a use-after-free (UAF) memory corruption.

Fix this issue by deferring the destruction of struct snd_uac_chip until
all references to the ALSA sound card are released. Register a custom
card->private_free callback (u_audio_card_free) during g_audio_setup()
that frees uac and its associated playback/capture request and ring
buffers only when the sound card reference count drops to zero.

Fixes: 6c67ed9ad9b8 ("usb: gadget: u_audio: don't let userspace block driver unbind")
Cc: stable@vger.kernel.org
Signed-off-by: Sonali Pradhan <sonalipradhan@google.com>
---
 drivers/usb/gadget/function/u_audio.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/drivers/usb/gadget/function/u_audio.c b/drivers/usb/gadget/function/u_audio.c
index ca26bf9c8040..f0ee83abcebc 100644
--- a/drivers/usb/gadget/function/u_audio.c
+++ b/drivers/usb/gadget/function/u_audio.c
@@ -1183,6 +1183,20 @@ static struct snd_kcontrol_new u_audio_controls[]  = {
 	},
 };
 
+static void u_audio_card_free(struct snd_card *card)
+{
+	struct snd_uac_chip *uac = card->private_data;
+
+	if (!uac)
+		return;
+
+	kfree(uac->p_prm.reqs);
+	kfree(uac->c_prm.reqs);
+	kfree(uac->p_prm.rbuf);
+	kfree(uac->c_prm.rbuf);
+	kfree(uac);
+}
+
 int g_audio_setup(struct g_audio *g_audio, const char *pcm_name,
 					const char *card_name)
 {
@@ -1262,6 +1276,8 @@ int g_audio_setup(struct g_audio *g_audio, const char *pcm_name,
 		goto fail;
 
 	uac->card = card;
+	card->private_data = uac;
+	card->private_free = u_audio_card_free;
 
 	/*
 	 * Create first PCM device
@@ -1430,6 +1446,8 @@ int g_audio_setup(struct g_audio *g_audio, const char *pcm_name,
 
 snd_fail:
 	snd_card_free(card);
+	return err;
+
 fail:
 	kfree(uac->p_prm.reqs);
 	kfree(uac->c_prm.reqs);
@@ -1455,12 +1473,6 @@ void g_audio_cleanup(struct g_audio *g_audio)
 	card = uac->card;
 	if (card)
 		snd_card_free_when_closed(card);
-
-	kfree(uac->p_prm.reqs);
-	kfree(uac->c_prm.reqs);
-	kfree(uac->p_prm.rbuf);
-	kfree(uac->c_prm.rbuf);
-	kfree(uac);
 }
 EXPORT_SYMBOL_GPL(g_audio_cleanup);
 
-- 
2.55.0.654.g21b8a5bc05-goog