[PATCH] spi: ch341: fix out-of-bounds memory access in ch341_transfer_one

[Tianchu Chen <tianchu.chen@linux.dev>]

From: Tianchu Chen <flynnnchen@tencent.com>

Discovered by Atuin - Automated Vulnerability Discovery Engine.

The 'len' variable is calculated as 'min(32, trans->len + 1)',
which includes the 1-byte command header.

When copying data from 'trans->tx_buf' to 'ch341->tx_buf + 1', using 'len'
as the length is incorrect because:

1. It causes an out-of-bounds read from 'trans->tx_buf' (which has size
   'trans->len', i.e., 'len - 1' in this context).
2. It can cause an out-of-bounds write to 'ch341->tx_buf' if 'len' is
   CH341_PACKET_LENGTH (32). Writing 32 bytes to ch341->tx_buf + 1
   overflows the buffer.

Fix this by copying 'len - 1' bytes.

Fixes: 8846739f52af ("spi: add ch341a usb2spi driver")
Signed-off-by: Tianchu Chen <flynnnchen@tencent.com>
---
 drivers/spi/spi-ch341.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/spi/spi-ch341.c b/drivers/spi/spi-ch341.c
index 46bc208f2..79d2f9ab4 100644
--- a/drivers/spi/spi-ch341.c
+++ b/drivers/spi/spi-ch341.c
@@ -78,7 +78,7 @@ static int ch341_transfer_one(struct spi_controller *host,
 
 	ch341->tx_buf[0] = CH341A_CMD_SPI_STREAM;
 
-	memcpy(ch341->tx_buf + 1, trans->tx_buf, len);
+	memcpy(ch341->tx_buf + 1, trans->tx_buf, len - 1);
 
 	ret = usb_bulk_msg(ch341->udev, ch341->write_pipe, ch341->tx_buf, len,
 			   NULL, CH341_DEFAULT_TIMEOUT);
-- 
2.39.5