[PATCH v2 1/2] drm/udl: reject short USB control transfers in udl_read_edid_block()

[Hui Peng <benquike@gmail.com>]

In udl_read_edid_block(), usb_control_msg() requests 2 bytes into
read_buff for each EDID byte, and read_buff[1] is stored into buf[i].
Currently the short-transfer check tests ret < 1 instead of ret != 2, so
if a short USB control transfer returns 1 byte, read_buff[1] is not
written by usb_control_msg() and retains the previous iteration's byte (or
uninitialized kmalloc(2) memory on the first iteration), which is then
copied into buf[i].

Change the check from ret < 1 to ret != 2 so any short USB control
transfer fails with -EIO.

Tested in QEMU against Linux 7.3.0-rc3 using dummy_hcd and raw-gadget
emulating a DisplayLink UDL USB device: on the unfixed kernel, when
usb_control_msg() returned 1 byte for an EDID read (short transfer),
udl_read_edid_block() accepted ret = 1 and stored stale read_buff[1]
data into the EDID buffer; whereas with this fix applied, ret != 2
correctly fails with -EIO and prevents EDID buffer corruption.

Fixes: 5320918b9a87 ("drm/udl: initial UDL driver (v4)")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@gmail.com>
---
Changes in v2:
- Split out as patch 1/2 as requested by Jani Nikula.
- Added testing details in QEMU on short USB EDID control transfers.

 drivers/gpu/drm/udl/udl_edid.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/udl/udl_edid.c b/drivers/gpu/drm/udl/udl_edid.c
index af4cff2a7c51..b7c462436573 100644
--- a/drivers/gpu/drm/udl/udl_edid.c
+++ b/drivers/gpu/drm/udl/udl_edid.c
@@ -36,7 +36,7 @@ static int udl_read_edid_block(void *data, u8 *buf, unsigned int block, size_t l
 		if (ret < 0) {
 			drm_err(dev, "Read EDID byte %zu failed err %x\n", i, ret);
 			goto err_drm_dev_exit;
-		} else if (ret < 1) {
+		} else if (ret != 2) {
 			ret = -EIO;
 			drm_err(dev, "Read EDID byte %zu failed\n", i);
 			goto err_drm_dev_exit;
-- 
2.55.0.1082.g2b9226bbc0-goog