[PATCH 1/1] usb: gadget: f_fs: Fix NULL pointer dereference in FUNCTIONFS_ENDPOINT_DESC

[zjamg <ndaugoing@gmail.com>]

When user space sets up FunctionFS with only full-speed descriptors
(e.g., flags = FUNCTIONFS_HAS_FS_DESC) and the gadget operates at a
higher speed (such as USB_SPEED_HIGH or USB_SPEED_SUPER),
config_ep_by_speed() detects that the function lacks descriptors for
the current speed, logs a warning, and falls back to full-speed
descriptors. The endpoint is then successfully enabled via
usb_ep_enable(), and epfile->ep becomes valid.

However, when user space subsequently issues the FUNCTIONFS_ENDPOINT_DESC
ioctl to query the endpoint descriptor, ffs_epfile_ioctl() selects the
descriptor index solely based on gadget->speed:

	switch (epfile->ffs->gadget->speed) {
	case USB_SPEED_SUPER:
	case USB_SPEED_SUPER_PLUS:
		desc_idx = 2;
		break;
	case USB_SPEED_HIGH:
		desc_idx = 1;
		break;
	default:
		desc_idx = 0;
	}
	desc = epfile->ep->descs[desc_idx];
	memcpy(&desc1, desc, desc->bLength);

Because user space only supplied full-speed descriptors,
epfile->ep->descs[1] is NULL. Dereferencing desc->bLength triggers an
immediate kernel NULL pointer dereference panic.

Fix this by falling back through lower speed descriptors if the current
speed descriptor was not supplied, matching the fallback logic in
config_ep_by_speed() and historical f_fs behavior. If still missing,
fall back to epfile->ep->ep->desc (the active descriptor), and return
-EINVAL safely if no valid descriptor is found. In addition, mark desc
as const to match struct usb_ep::desc and avoid discarding qualifiers.

Fixes: c559a3534109 ("usb: gadget: f_fs: add ioctl returning ep descriptor")
Cc: stable@vger.kernel.org
Signed-off-by: zjamg <ndaugoing@gmail.com>
---
 drivers/usb/gadget/function/f_fs.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
index 43962e05eacf..c64a268e98a4 100644
--- a/drivers/usb/gadget/function/f_fs.c
+++ b/drivers/usb/gadget/function/f_fs.c
@@ -1877,8 +1877,9 @@ static long ffs_epfile_ioctl(struct file *file, unsigned code,
 		break;
 	case FUNCTIONFS_ENDPOINT_DESC:
 	{
+		const struct usb_endpoint_descriptor *desc;
+		struct usb_endpoint_descriptor desc1;
 		int desc_idx;
-		struct usb_endpoint_descriptor desc1, *desc;
 
 		switch (epfile->ffs->gadget->speed) {
 		case USB_SPEED_SUPER:
@@ -1892,7 +1893,17 @@ static long ffs_epfile_ioctl(struct file *file, unsigned code,
 			desc_idx = 0;
 		}
 
-		desc = epfile->ep->descs[desc_idx];
+		do {
+			desc = epfile->ep->descs[desc_idx];
+		} while (!desc && --desc_idx >= 0);
+
+		if (!desc)
+			desc = epfile->ep->ep->desc;
+		if (!desc) {
+			ret = -EINVAL;
+			break;
+		}
+
 		memcpy(&desc1, desc, desc->bLength);
 
 		spin_unlock_irq(&epfile->ffs->eps_lock);
-- 
2.53.0