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

[Zhang Cen <rollkingzzc@gmail.com>]

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