Re: [PATCH] usb: gadget: f_fs: Fix kernel panic for SuperSpeed

[Felipe Balbi]

Hi Jim,

Jim Lin <jilin@xxxxxxxxxx> writes:
> Android N adds os_desc_compat in v2_descriptor by init_functionfs()
> (system/core/adb/usb_linux_client.cpp) to support automatic install
> of MTP driver on Windows for USB device mode.
>
> Current __ffs_data_do_os_desc() of f_fs.c will check reserved1 field
> and return -EINVAL.
> This results in a second adb_write of usb_linux_client.cpp
> (system/core/adb/) which doesn't have ss_descriptors filled.
> Then later kernel_panic (composite.c) occurs when ss_descriptors
> as a pointer with NULL is being accessed.

where exactly in composite.c are we dereferencing a NULL pointer ?

Is this happening on set_config() ? If that's the case, why is
gadget->speed set to USB_SPEED_SUPER to start with ? Your controller
should already have negotiated highspeed which means
function_descriptors() should have returned highspeed descriptors, not a
NULL superspeed.

Care to explain why you haven't negotiated Highspeed ? The only thing I
can think of is that you're using a Superspeed-capable peripheral
controller (dwc3?) with maximum-speed set to Superspeed, with a
Superspeed-capable cable connected to an XHCI PC, but loading a
high-speed gadget driver (which you got from Android, written with f_fs)
and this gadget doesn't tell composite that its maximum speed is
Highspeed, instead of super-speed.

We can add a check, sure, to avoid a kernel oops; however, you should
really fix up the gadget implementation and/or set dwc3's maximum-speed
property accordingly.

Can you check if this patch makes your scenario work while still being
fully functional ?

diff --git a/drivers/usb/gadget/composite.c b/drivers/usb/gadget/composite.c
index de9ffd60fcfa..3d3cdc5ed20d 100644
--- a/drivers/usb/gadget/composite.c
+++ b/drivers/usb/gadget/composite.c
@@ -66,20 +66,36 @@ function_descriptors(struct usb_function *f,
 {
 	struct usb_descriptor_header **descriptors;
 
+	/*
+	 * NOTE: we try to help gadget drivers which might not be setting
+	 * max_speed appropriately.
+	 */
+
 	switch (speed) {
 	case USB_SPEED_SUPER_PLUS:
 		descriptors = f->ssp_descriptors;
-		break;
+		if (descriptors)
+			break;
+		/* FALLTHROUGH */
 	case USB_SPEED_SUPER:
 		descriptors = f->ss_descriptors;
-		break;
+		if (descriptors)
+			break;
+		/* FALLTHROUGH */
 	case USB_SPEED_HIGH:
 		descriptors = f->hs_descriptors;
-		break;
+		if (descriptors)
+			break;
+		/* FALLTHROUGH */
 	default:
 		descriptors = f->fs_descriptors;
 	}
 
+	/*
+	 * if we can't find any descriptors at all, then this gadget deserves to
+	 * Oops with a NULL pointer dereference
+	 */
+
 	return descriptors;
 }
 
-- 
balbi

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