[PATCH v2] HID: bpf: add __counted_by_ptr attribute to device_data

[Bill Wendling <morbo@google.com>]

The 'struct hid_bpf' contains a 'device_data' pointer field (of type
'u8 *') and an 'allocated_data' field (of type 'u32') that specifies the
size in bytes of the allocated memory for 'device_data'. Since
'device_data' is a pointer to 'u8' (elements of size 1 byte),
'allocated_data' represents the exact count of elements allocated for
'device_data'.

Annotate the 'device_data' field of 'struct hid_bpf' with the
'__counted_by_ptr' attribute, pointing to 'allocated_data'. This enables
bounds-checking sanitizers (like KASAN and UBSAN) to detect
out-of-bounds accesses to 'device_data'.

Because the count 'allocated_data' is always set before any access and
accurately tracks the allocated buffer size at all times, adding
'__counted_by_ptr' will not cause runtime panics or false-positive
bounds checks.

Cc: codemender-patching+linux@google.com
Assisted-by: LLM
Signed-off-by: Bill Wendling <morbo@google.com>
---
v2: Reorder the assignment of the buffer and the count field. It won't
    generate an exception during execution, but it's a good coding
    habit that also satisfies LLMs' paranoia.
---
 drivers/hid/bpf/hid_bpf_dispatch.c |  2 +-
 include/linux/hid_bpf.h            | 10 ++++++----
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/drivers/hid/bpf/hid_bpf_dispatch.c b/drivers/hid/bpf/hid_bpf_dispatch.c
index d46779b63660..fb302d0b9797 100644
--- a/drivers/hid/bpf/hid_bpf_dispatch.c
+++ b/drivers/hid/bpf/hid_bpf_dispatch.c
@@ -255,8 +255,8 @@ static int __hid_bpf_allocate_data(struct hid_device *hdev, u8 **data, u32 *size
 	if (!alloc_data)
 		return -ENOMEM;
 
-	*data = alloc_data;
 	*size = alloc_size;
+	*data = alloc_data;
 
 	return 0;
 }
diff --git a/include/linux/hid_bpf.h b/include/linux/hid_bpf.h
index 19fffa4574a4..f45fb9cccece 100644
--- a/include/linux/hid_bpf.h
+++ b/include/linux/hid_bpf.h
@@ -185,10 +185,12 @@ struct hid_bpf_ops {
 
 /* stored in each device */
 struct hid_bpf {
-	u8 *device_data;		/* allocated when a bpf program of type
-					 * SEC(f.../hid_bpf_device_event) has been attached
-					 * to this HID device
-					 */
+	/*
+	 * allocated when a bpf program of type
+	 * SEC(f.../hid_bpf_device_event) has been attached
+	 * to this HID device
+	 */
+	u8 *device_data __counted_by_ptr(allocated_data);
 	u32 allocated_data;
 	bool destroyed;			/* prevents the assignment of any progs */
 
-- 
2.55.0.1082.g2b9226bbc0-goog