[PATCH] usb: gadget: midi2: Fix null-pointer dereference in f_midi2_free_ep_reqs

["syzbot" <syzbot@kernel.org>]

From: Aleksandr Nogikh <nogikh@google.com>

A null-pointer dereference occurs in f_midi2_free_ep_reqs() when attempting
to clean up an endpoint that was never initialized.

When configuring the MIDI 2.0 gadget via configfs and setting the block
direction to SNDRV_UMP_DIR_INPUT, the initialization of the midi1_ep_out
endpoint is explicitly skipped during the gadget bind phase
(f_midi2_bind()). As a result, the usb_ep->card field remains NULL.

Later, when the host sets the alternate setting, f_midi2_set_alt()
unconditionally stops both the IN and OUT endpoints by calling
f_midi2_stop_eps(), which in turn calls f_midi2_free_ep_reqs() for both
endpoints. When f_midi2_free_ep_reqs() is called for the uninitialized
midi1_ep_out, it attempts to dereference usb_ep->card to determine the
number of requests to free, leading to a crash.

Fix this by using usb_ep->num_reqs instead of usb_ep->card->info.num_reqs
in f_midi2_free_ep_reqs(). usb_ep->num_reqs is correctly set during
f_midi2_init_ep() and remains 0 if the endpoint was never initialized,
safely avoiding the loop. For consistency, apply the same change to
f_midi2_alloc_ep_reqs().

Oops: general protection fault, probably for non-canonical address
0xdffffc00000000ee: 0000 [#1] SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x0000000000000770-0x0000000000000777]
...
RIP: 0010:f_midi2_free_ep_reqs drivers/usb/gadget/function/f_midi2.c:1166
[inline]
RIP: 0010:f_midi2_stop_eps+0x28e/0x4d0
drivers/usb/gadget/function/f_midi2.c:1246
...
Call Trace:
 <TASK>
 f_midi2_set_alt+0x11c/0xf00 drivers/usb/gadget/function/f_midi2.c:1296
 composite_setup+0x1ffd/0x3480 drivers/usb/gadget/composite.c:1933
 configfs_composite_setup+0xbd/0x100 drivers/usb/gadget/configfs.c:1877

Fixes: 8b645922b223 ("usb: gadget: Add support for USB MIDI 2.0 function driver")
Assisted-by: Gemini:gemini-3.5-flash Gemini:gemini-3.1-pro-preview syzbot
Reported-by: syzbot+bbb6dad313f4aaa8da6b@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=bbb6dad313f4aaa8da6b
Link: https://syzkaller.appspot.com/ai_job?id=8ce30b1a-8cf7-4e38-bcf7-1f69e6f6313f
Signed-off-by: Aleksandr Nogikh <nogikh@google.com>

---
diff --git a/drivers/usb/gadget/function/f_midi2.c b/drivers/usb/gadget/function/f_midi2.c
index 19fdac024..5e54cf994 100644
--- a/drivers/usb/gadget/function/f_midi2.c
+++ b/drivers/usb/gadget/function/f_midi2.c
@@ -1145,7 +1145,7 @@ static int f_midi2_alloc_ep_reqs(struct f_midi2_usb_ep *usb_ep)
 	if (!usb_ep->reqs)
 		return -EINVAL;
 
-	for (i = 0; i < midi2->info.num_reqs; i++) {
+	for (i = 0; i < usb_ep->num_reqs; i++) {
 		if (usb_ep->reqs[i].req)
 			continue;
 		usb_ep->reqs[i].req = alloc_ep_req(usb_ep->usb_ep,
@@ -1160,10 +1160,9 @@ static int f_midi2_alloc_ep_reqs(struct f_midi2_usb_ep *usb_ep)
 /* Free allocated requests */
 static void f_midi2_free_ep_reqs(struct f_midi2_usb_ep *usb_ep)
 {
-	struct f_midi2 *midi2 = usb_ep->card;
 	int i;
 
-	for (i = 0; i < midi2->info.num_reqs; i++) {
+	for (i = 0; i < usb_ep->num_reqs; i++) {
 		if (!usb_ep->reqs[i].req)
 			continue;
 		free_ep_req(usb_ep->usb_ep, usb_ep->reqs[i].req);


base-commit: f5098b6bae761e346ebcd9da7f95622c04733cff
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