[PATCH] USB: serial: cypress_m8: validate interrupt packet headers

[PATCH] USB: serial: cypress_m8: validate interrupt packet headers

[Zhang Cen]

cypress_read_int_callback() parses the interrupt-in buffer according to
the selected Cypress packet format. Format 1 has a two-byte status/count
header and format 2 has a one-byte combined status/count header. The
usb-serial core sizes the interrupt-in buffer from the endpoint
descriptor's wMaxPacketSize, and successful interrupt transfers can
complete short when URB_SHORT_NOT_OK is not set.

Check that both the allocated URB buffer and completed packet contain the
selected header before reading it. Malformed short reports are ignored and
the interrupt URB is resubmitted through the existing retry path,
preventing out-of-bounds and stale header-byte reads.

KASAN report as below:
KASAN slab-out-of-bounds in cypress_read_int_callback+0x240/0x7f0
RIP: 0010:kasan_check_range+0x67/0x1b0
Read of size 1
Call trace:
  dump_stack_lvl+0x66/0xa0 (?:?)
  print_report+0xce/0x630 (?:?)
  cypress_read_int_callback() (drivers/usb/serial/cypress_m8.c:1009)
  srso_alias_return_thunk+0x5/0xfbef5 (?:?)
  __virt_addr_valid+0x188/0x320 (?:?)
  kasan_report+0xe0/0x110 (?:?)
  __usb_hcd_giveback_urb+0x103/0x1d0 (?:?)
  __usb_hcd_giveback_urb+0xf3/0x1d0 (?:?)
  __usb_hcd_giveback_urb+0x112/0x1d0 (?:?)
  dummy_timer+0xaaa/0x19a0 (?:?)
  mark_held_locks+0x40/0x70 (?:?)
  _raw_spin_unlock_irqrestore+0x44/0x60 (?:?)
  lockdep_hardirqs_on_prepare+0xb7/0x1a0 (?:?)
  __hrtimer_run_queues+0x102/0x510 (?:?)
  hrtimer_run_softirq+0xd0/0x130 (?:?)
  handle_softirqs+0x155/0x650 (?:?)
  __irq_exit_rcu+0xc4/0x160 (?:?)
  irq_exit_rcu+0xe/0x20 (?:?)
  sysvec_apic_timer_interrupt+0x6c/0x80 (?:?)
  asm_sysvec_apic_timer_interrupt+0x1a/0x20 (?:?)

Fixes: 3416eaa1f8f8 ("USB: cypress_m8: Packet format is separate from characteristic size")
Assisted-by: Codex:gpt-5.5
Signed-off-by: Zhang Cen <rollkingzzc@gmail.com>
---
 drivers/usb/serial/cypress_m8.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/drivers/usb/serial/cypress_m8.c b/drivers/usb/serial/cypress_m8.c
index afff1a0f4298b..50c6abc69e756 100644
--- a/drivers/usb/serial/cypress_m8.c
+++ b/drivers/usb/serial/cypress_m8.c
@@ -1016,6 +1016,7 @@ static void cypress_read_int_callback(struct urb *urb)
 	unsigned long flags;
 	char tty_flag = TTY_NORMAL;
 	int bytes = 0;
+	int header_size = 0;
 	int result;
 	int i = 0;
 	int status = urb->status;
@@ -1060,18 +1061,32 @@ static void cypress_read_int_callback(struct urb *urb)
 	default:
 	case packet_format_1:
 		/* This is for the CY7C64013... */
+		header_size = 2;
+		if (result < header_size ||
+		    urb->transfer_buffer_length < header_size)
+			break;
 		priv->current_status = data[0] & 0xF8;
 		bytes = data[1] + 2;
 		i = 2;
 		break;
 	case packet_format_2:
 		/* This is for the CY7C63743... */
+		header_size = 1;
+		if (result < header_size ||
+		    urb->transfer_buffer_length < header_size)
+			break;
 		priv->current_status = data[0] & 0xF8;
 		bytes = (data[0] & 0x07) + 1;
 		i = 1;
 		break;
 	}
 	spin_unlock_irqrestore(&priv->lock, flags);
+	if (result < header_size || urb->transfer_buffer_length < header_size) {
+		dev_dbg(dev,
+			"%s - short packet header - received %d bytes but buffer has %d bytes\n",
+			__func__, result, urb->transfer_buffer_length);
+		goto continue_read;
+	}
 	if (result < bytes) {
 		dev_dbg(dev,
 			"%s - wrong packet size - received %d bytes but packet said %d bytes\n",
-- 
2.43.0

Re: [PATCH] USB: serial: cypress_m8: validate interrupt packet headers

[Johan Hovold]

On Fri, May 22, 2026 at 12:11:17AM +0800, Zhang Cen wrote:
> cypress_read_int_callback() parses the interrupt-in buffer according to
> the selected Cypress packet format. Format 1 has a two-byte status/count
> header and format 2 has a one-byte combined status/count header. The
> usb-serial core sizes the interrupt-in buffer from the endpoint
> descriptor's wMaxPacketSize, and successful interrupt transfers can
> complete short when URB_SHORT_NOT_OK is not set.
> 
> Check that both the allocated URB buffer and completed packet contain the
> selected header before reading it. Malformed short reports are ignored and
> the interrupt URB is resubmitted through the existing retry path,
> preventing out-of-bounds and stale header-byte reads.

Good catch.

> KASAN report as below:
> KASAN slab-out-of-bounds in cypress_read_int_callback+0x240/0x7f0
> RIP: 0010:kasan_check_range+0x67/0x1b0
> Read of size 1
> Call trace:
>   dump_stack_lvl+0x66/0xa0 (?:?)
>   print_report+0xce/0x630 (?:?)
>   cypress_read_int_callback() (drivers/usb/serial/cypress_m8.c:1009)
>   srso_alias_return_thunk+0x5/0xfbef5 (?:?)
>   __virt_addr_valid+0x188/0x320 (?:?)
>   kasan_report+0xe0/0x110 (?:?)
>   __usb_hcd_giveback_urb+0x103/0x1d0 (?:?)
>   __usb_hcd_giveback_urb+0xf3/0x1d0 (?:?)
>   __usb_hcd_giveback_urb+0x112/0x1d0 (?:?)
>   dummy_timer+0xaaa/0x19a0 (?:?)

You can trim call traces like this one from here (dummy_timer).

>   mark_held_locks+0x40/0x70 (?:?)
>   _raw_spin_unlock_irqrestore+0x44/0x60 (?:?)
>   lockdep_hardirqs_on_prepare+0xb7/0x1a0 (?:?)
>   __hrtimer_run_queues+0x102/0x510 (?:?)
>   hrtimer_run_softirq+0xd0/0x130 (?:?)
>   handle_softirqs+0x155/0x650 (?:?)
>   __irq_exit_rcu+0xc4/0x160 (?:?)
>   irq_exit_rcu+0xe/0x20 (?:?)
>   sysvec_apic_timer_interrupt+0x6c/0x80 (?:?)
>   asm_sysvec_apic_timer_interrupt+0x1a/0x20 (?:?)
> 
> Fixes: 3416eaa1f8f8 ("USB: cypress_m8: Packet format is separate from characteristic size")
> Assisted-by: Codex:gpt-5.5
> Signed-off-by: Zhang Cen <rollkingzzc@gmail.com>
> ---
>  drivers/usb/serial/cypress_m8.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/drivers/usb/serial/cypress_m8.c b/drivers/usb/serial/cypress_m8.c
> index afff1a0f4298b..50c6abc69e756 100644
> --- a/drivers/usb/serial/cypress_m8.c
> +++ b/drivers/usb/serial/cypress_m8.c
> @@ -1016,6 +1016,7 @@ static void cypress_read_int_callback(struct urb *urb)
>  	unsigned long flags;
>  	char tty_flag = TTY_NORMAL;
>  	int bytes = 0;
> +	int header_size = 0;
>  	int result;
>  	int i = 0;
>  	int status = urb->status;
> @@ -1060,18 +1061,32 @@ static void cypress_read_int_callback(struct urb *urb)
>  	default:
>  	case packet_format_1:
>  		/* This is for the CY7C64013... */
> +		header_size = 2;
> +		if (result < header_size ||
> +		    urb->transfer_buffer_length < header_size)

As with you previous patch, there is no need to check the
transfer_buffer_length (here or below).

> +			break;
>  		priv->current_status = data[0] & 0xF8;
>  		bytes = data[1] + 2;
>  		i = 2;

The variable i already holds the header size, so even if it's not
very well named you should reuse that one after moving the assignment.

>  		break;
>  	case packet_format_2:
>  		/* This is for the CY7C63743... */
> +		header_size = 1;
> +		if (result < header_size ||
> +		    urb->transfer_buffer_length < header_size)
> +			break;
>  		priv->current_status = data[0] & 0xF8;
>  		bytes = (data[0] & 0x07) + 1;
>  		i = 1;
>  		break;
>  	}
>  	spin_unlock_irqrestore(&priv->lock, flags);
> +	if (result < header_size || urb->transfer_buffer_length < header_size) {
> +		dev_dbg(dev,
> +			"%s - short packet header - received %d bytes but buffer has %d bytes\n",
> +			__func__, result, urb->transfer_buffer_length);

Just say "short packet received" and possibly include result.

> +		goto continue_read;
> +	}
>  	if (result < bytes) {
>  		dev_dbg(dev,
>  			"%s - wrong packet size - received %d bytes but packet said %d bytes\n",

Johan