Re: [PATCH] HID: hyperv: fix fortify write overflow in KUnit device info tests
From: Benjamin Tissoires <bentiss@kernel.org>
Date: 2026-09-07 13:19:37
Also in:
linux-hyperv, lkml
On Sep 07 2026, Baoli Zhang wrote:
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 <redacted>
Please always check is a patch is not already pending in linux-next or
in mainline linux.
d0ad81b2b5fe ("HID: hyperv: make pointer arithmetics understandable for
FORTIFY_SOURCE") already fixes this very same issue.
Cheers,
Benjamin
quoted hunk ↗ jump to hunk
--- 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