[PATCH v4 1/2] Input: synaptics-rmi4 - fix irq[] overrun with 7 interrupt sources

rmi_read_pdt_entry() takes the interrupt source count straight out of the
Page Description Table entry the device supplies:

	entry->interrupt_source_count = buf[4] & RMI_PDT_INT_SOURCE_COUNT_MASK;

RMI_PDT_INT_SOURCE_COUNT_MASK is 0x07, so the value can be 7, and
rmi_create_function() copies it verbatim into fn->num_of_irqs.  But
struct rmi_function declares

	int irq[RMI_FN_MAX_IRQS];

with RMI_FN_MAX_IRQS == 6, and both rmi_create_function_irq() and
rmi_unregister_function() index that array up to fn->num_of_irqs.  A
device declaring 7 interrupt sources for a function that has a handler --
F01 always does -- makes the driver write irq[6], which is the storage of
the following member, unsigned int irq_pos.  The function's position in
the interrupt bitmap then holds a Linux virq number, and that value feeds
set_bit(fn->irq_pos, ...) in rmi_f11_probe()/rmi_f12_probe() and the
irq_dispose_mapping() loop on teardown.

UBSAN reports every store in the loop body and the read on the unregister
path:

  UBSAN: array-index-out-of-bounds in drivers/input/rmi4/rmi_bus.c:183:10
  index 6 is out of range for type 'int [6]'
  Workqueue: events uhid_device_add_worker
   dump_stack_lvl+0x64/0x80
   __ubsan_handle_out_of_bounds+0xc8/0x100
   rmi_function_probe+0x1c1/0x210 [rmi_core]
  UBSAN: array-index-out-of-bounds in drivers/input/rmi4/rmi_bus.c:186:28
  UBSAN: array-index-out-of-bounds in drivers/input/rmi4/rmi_bus.c:187:35
  UBSAN: array-index-out-of-bounds in drivers/input/rmi4/rmi_bus.c:189:32
  UBSAN: array-index-out-of-bounds in drivers/input/rmi4/rmi_bus.c:191:54
  UBSAN: array-index-out-of-bounds in drivers/input/rmi4/rmi_bus.c:282:30

Size the array to match the three bit field that feeds it.  Clamping
num_of_irqs instead would silently drop an interrupt source a device is
allowed to declare, and would desynchronise irq_pos for every function
created after it.

Reproduced with an emulated RMI4 device that publishes a single F01 PDT
entry with interrupt_source_count = 7, driven over /dev/uhid and again
over dummy_hcd plus raw-gadget, on v6.12.69 and v6.12.105 with
CONFIG_UBSAN_BOUNDS=y.  No reports after this change, and the same device
now probes normally.

Fixes: 24d28e4f1271 ("Input: synaptics-rmi4 - convert irq distribution to irq_domain")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-5
Assisted-by: GLM:glm-5.3
Signed-off-by: Wei Jie Law <redacted>
---
Changes in v4:
 - No code change: adds the Assisted-by tags required by
   Documentation/process/coding-assistants.rst.  The diff has been
   unchanged since v1.

 drivers/input/rmi4/rmi_bus.h | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/input/rmi4/rmi_bus.h b/drivers/input/rmi4/rmi_bus.h
index 90122df21f74..faf2ebb00d52 100644
--- a/drivers/input/rmi4/rmi_bus.h
+++ b/drivers/input/rmi4/rmi_bus.h

@@ -12,10 +12,13 @@
 struct rmi_device;
 
 /*
- * The interrupt source count in the function descriptor can represent up to
- * 6 interrupt sources in the normal manner.
+ * The interrupt source count in the function descriptor is a three bit field
+ * (RMI_PDT_INT_SOURCE_COUNT_MASK), so a device can legitimately declare up to
+ * 7 interrupt sources for a single function.  irq[] must be able to hold all
+ * of them: rmi_create_function_irq() and rmi_unregister_function() both walk
+ * it up to fn->num_of_irqs.
  */
-#define RMI_FN_MAX_IRQS	6
+#define RMI_FN_MAX_IRQS	7
 
 /**
  * struct rmi_function - represents the implementation of an RMI4

-- 
2.43.0