Date:   Tue, 23 Aug 2016 02:36:30 +0200
From:   Clemens Gruber <clemens.gruber@...ruber.com>
To:     linux-usb@...r.kernel.org
Cc:     Peter Chen <Peter.Chen@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Subject: chipidea: udc: kernel panic in isr_setup_status_phase

Hi,

I am using an i.MX6Q embedded board, acting as a (ethernet) gadget with
RNDIS function, connected over an USB OTG cable to a PC.
Most of the time it works fine, but in some mysterious circumstances,
a kernel panic occurs, just after attaching the OTG cable, connecting it
to the other machine:

[   54.012989] Unable to handle kernel NULL pointer dereference at virtual address 00000020
[   54.021099] pgd = 80004000
[   54.023816] [00000020] *pgd=00000000
[   54.027422] Internal error: Oops: 817 [#1] PREEMPT SMP ARM
[   54.032915] Modules linked in:
[   54.035998] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.8.0-rc3-00017-g336bc4a #315
[   54.043662] Hardware name: Freescale i.MX6 Quad/DualLite (Device Tree)
[   54.050196] task: 80b05f80 task.stack: 80b00000
[   54.054744] PC is at isr_setup_status_phase+0x1c/0x40
[   54.059805] LR is at 0xbe570890
[   54.062957] pc : [<804ac464>]    lr : [<be570890>]    psr: 200e0193
[   54.062957] sp : 80b01e10  ip : be570570  fp : be570890
[   54.074442] r10: be5eeebc  r9 : be570010  r8 : be5eeebc
[   54.079673] r7 : be5708d0  r6 : be5eee80  r5 : be7fcf40  r4 : 00000001
[   54.086206] r3 : be571010  r2 : 804ab368  r1 : 00000000  r0 : be570010
[   54.092742] Flags: nzCv  IRQs off  FIQs on  Mode SVC_32  ISA ARM  Segment none
[   54.099972] Control: 10c5387d  Table: 4e34404a  DAC: 00000051
[   54.105723] Process swapper/0 (pid: 0, stack limit = 0x80b00210)
(snip)
[   54.247100] [<804ac464>] (isr_setup_status_phase) from [<804acbbc>] (isr_tr_complete_handler+0x734/0x98c)
[   54.256680] [<804acbbc>] (isr_tr_complete_handler) from [<804acfc0>] (udc_irq+0x1ac/0x318)
[   54.264964] [<804acfc0>] (udc_irq) from [<8018ba28>] (__handle_irq_event_percpu+0x9c/0x128)
[   54.273330] [<8018ba28>] (__handle_irq_event_percpu) from [<8018bae0>] (handle_irq_event_percpu+0x2c/0x7c)
[   54.282995] [<8018bae0>] (handle_irq_event_percpu) from [<8018bb68>] (handle_irq_event+0x38/0x5c)
[   54.291880] [<8018bb68>] (handle_irq_event) from [<8018f2cc>] (handle_fasteoi_irq+0xd0/0x1bc)
[   54.300418] [<8018f2cc>] (handle_fasteoi_irq) from [<8018afb0>] (generic_handle_irq+0x24/0x34)
[   54.309042] [<8018afb0>] (generic_handle_irq) from [<8018b2dc>] (__handle_domain_irq+0x7c/0xec)
[   54.317754] [<8018b2dc>] (__handle_domain_irq) from [<80101524>] (gic_handle_irq+0x38/0x74)
[   54.326119] [<80101524>] (gic_handle_irq) from [<8010ccb0>] (__irq_svc+0x70/0xb0)
(snip)

After looking through the isr_setup_status_phase disassembly, I found
that ci->status must have been NULL and dereferencing it in
ci->status->context = ci; triggered the panic.

The interrupt was a USBINT (UI bit was set) and isr_tr_complete_handler
was called from udc_irq.
In the IMX6DQRM I read about the UI bit: "This bit is also set by the
Host/Device Controller when a short packet is detected." and about
USBERRINT / UEI bit: "This bit is set along with the USBINT bit, if the
TD on which the error interrupt occurred also had its interrupt on
complete (IOC) bit set." (page 5494)

However, we do not check for UEI in udc_irq.
Could this be the cause of this error?
Should we only call isr_tr_complete_handler if UI && !UEI ?

Or would adding a check for ci->status == NULL in isr_setup-status_phase
and returning an error code also be a good idea?

Do you have an idea what's going on there and why ci->status is NULL?

Regards,
Clemens

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