[PATCH v2 2/3] HID: mcp2221: clear rxbuf after I2C/SMBus transfer completes

mcp_i2c_smbus_read() stores the caller-supplied buffer pointer in
mcp->rxbuf for the duration of a transfer but never clears it when the
transfer finishes or times out. Once the caller frees or reuses the
buffer, mcp->rxbuf becomes a dangling pointer. A delayed or spurious
MCP2221_I2C_GET_DATA report can then drive mcp2221_raw_event() to
memcpy device data into the freed memory, causing a write
use-after-free.

Route all return paths through a single exit point that clears
mcp->rxbuf and mcp->rxbuf_size, so that the existing !mcp->rxbuf guard
in the raw_event handler can reject any report arriving after the
transfer has ended.

Fixes: 67a95c21463d ("HID: mcp2221: add usb to i2c-smbus host bridge")
Cc: [email protected]
Signed-off-by: Jiangshan Yi <[email protected]>
---
 drivers/hid/hid-mcp2221.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/hid/hid-mcp2221.c b/drivers/hid/hid-mcp2221.c
index 5c7fc56..9e03d1f 100644
--- a/drivers/hid/hid-mcp2221.c
+++ b/drivers/hid/hid-mcp2221.c
@@ -343,7 +343,7 @@ static int mcp_i2c_smbus_read(struct mcp2221 *mcp,

 	ret = mcp_send_data_req_status(mcp, mcp->txbuf, 4);
 	if (ret)
-		return ret;
+		goto out;

 	mcp->rxbuf_idx = 0;

@@ -365,7 +365,7 @@ static int mcp_i2c_smbus_read(struct mcp2221 *mcp,
 			} else {
 				usleep_range(980, 1000);
 				mcp_cancel_last_cmd(mcp);
-				return ret;
+				goto out;
 			}
 		} else {
 			retries = 0;
@@ -375,6 +375,10 @@ static int mcp_i2c_smbus_read(struct mcp2221 *mcp,
 	usleep_range(980, 1000);
 	ret = mcp_chk_last_cmd_status_free_bus(mcp);

+out:
+	mcp->rxbuf = NULL;
+	mcp->rxbuf_size = 0;
+
 	return ret;
 }
 
-- 
2.25.1