[PATCH] HID: hyperv: fix fortify write overflow in KUnit device info tests

[Baoli Zhang]

allyesconfig fails to build:

  In function 'fortify_memset_chk',
      inlined from 'mousevsc_device_info_valid_descriptor':
  ./include/linux/fortify-string.h:430:25: error: call to
  '__write_overflow_field' declared with attribute warning: detected
  write beyond size of field (1st parameter); maybe use struct_group()?

Both tests place a report descriptor in bytes allocated past
struct synthhid_device_info, then memset() it, and derive its address
from the hid_descriptor member:

  report = ((u8 *)&info->hid_descriptor) + info->hid_descriptor.bLength;

Because that expression is a member reference, the fortify checks bound
the write by sizeof(info->hid_descriptor). bLength is exactly that size
here, so the remaining room in the member is zero and every write is
flagged, even though the memory really came from the caller's
kunit_kzalloc(sizeof(*info) + N).

Compute the address from the start of info instead, mirroring the
desc_offset arithmetic that mousevsc_on_receive_device_info() itself
uses. Deriving from info gives the whole allocation as the object, so
the writes are no longer attributed to a single member. The result is
the same address: (u8 *)&info->hid_descriptor is by definition
(u8 *)info + offsetof(struct synthhid_device_info, hid_descriptor), so
test behaviour is unchanged.

sizeof(*info) deliberately is not used for this: hid_descriptor is
__packed while synthhid_device_info is not, so the struct may carry
trailing padding and the report would land at the wrong offset.

Fixes: 83df7b5fa6735 ("HID: hyperv: add KUnit coverage for device info bounds")
Signed-off-by: Baoli Zhang <baoli.zhang@xxxxxxxxxxxxxxx>
---
 drivers/hid/hid-hyperv.c | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/drivers/hid/hid-hyperv.c b/drivers/hid/hid-hyperv.c
index 6579bd19da13a..f1042b36d10b7 100644
--- a/drivers/hid/hid-hyperv.c
+++ b/drivers/hid/hid-hyperv.c
@@ -657,6 +657,25 @@ static struct mousevsc_dev *mousevsc_kunit_alloc_dev(struct kunit *test)
 	return input_dev;
 }
 
+/*
+ * Address of the report descriptor that mousevsc_on_receive_device_info()
+ * expects immediately after the HID descriptor.
+ *
+ * Computed from the start of @info, mirroring the desc_offset arithmetic in
+ * mousevsc_on_receive_device_info(), rather than from &info->hid_descriptor.
+ * The report lives in the extra bytes the caller allocated past the struct, so
+ * deriving it from the member would leave the fortify checks bounding writes
+ * by sizeof(info->hid_descriptor) and reject them. Note sizeof(*info) cannot
+ * be used instead: hid_descriptor is packed while synthhid_device_info is not,
+ * so the struct may carry trailing padding.
+ */
+static u8 *mousevsc_kunit_report_desc(struct synthhid_device_info *info)
+{
+	return (u8 *)info +
+	       offsetof(struct synthhid_device_info, hid_descriptor) +
+	       info->hid_descriptor.bLength;
+}
+
 static void mousevsc_device_info_zero_blength(struct kunit *test)
 {
 	struct synthhid_device_info *info;
@@ -687,7 +706,7 @@ static void mousevsc_device_info_valid_descriptor(struct kunit *test)
 
 	info->hid_descriptor.bLength = sizeof(struct hid_descriptor);
 	info->hid_descriptor.rpt_desc.wDescriptorLength = cpu_to_le16(4);
-	report = ((u8 *)&info->hid_descriptor) + info->hid_descriptor.bLength;
+	report = mousevsc_kunit_report_desc(info);
 	memset(report, 0x42, 4);
 
 	mousevsc_on_receive_device_info(input_dev, info, sizeof(*info) + 4);
@@ -713,7 +732,7 @@ static void mousevsc_device_info_report_desc_oob(struct kunit *test)
 
 	info->hid_descriptor.bLength = sizeof(struct hid_descriptor);
 	info->hid_descriptor.rpt_desc.wDescriptorLength = cpu_to_le16(64);
-	report = ((u8 *)&info->hid_descriptor) + info->hid_descriptor.bLength;
+	report = mousevsc_kunit_report_desc(info);
 	memset(report, 0x42, 8);
 
 	mousevsc_on_receive_device_info(input_dev, info, sizeof(*info) + 8);
-- 
2.43.0