[PATCH] Input: rmi_smbus - fix out-of-bounds read in rmi_smb_write_block()

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

When chunking writes into SMBus blocks in rmi_smb_write_block(), the
loop calculates block_len using the original total length (len) instead
of the remaining length (cur_len).

If len is greater than 32 bytes (SMB_MAX_COUNT), block_len remains 32
for every iteration, even on the final partial chunk where fewer than 32
bytes remain. This causes smb_block_write() to read 32 bytes from the
advanced data buffer pointer, reading past the end of the input buffer.

Fix this by calculating block_len using cur_len and advancing the buffer
and address pointers by block_len.

Fixes: 82264d0cf7aef ("Input: synaptics-rmi4 - add SMBus support")
Cc: stable@vger.kernel.org
Reported-by: sashiko-bot@kernel.org
Assisted-by: Antigravity:gemini-3.6-flash
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
---
 drivers/input/rmi4/rmi_smbus.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/input/rmi4/rmi_smbus.c b/drivers/input/rmi4/rmi_smbus.c
index 6de68c602558..d799405fbd33 100644
--- a/drivers/input/rmi4/rmi_smbus.c
+++ b/drivers/input/rmi4/rmi_smbus.c
@@ -140,7 +140,7 @@ static int rmi_smb_write_block(struct rmi_transport_dev *xport, u16 rmiaddr,
 	u8 commandcode;
 	struct rmi_smb_xport *rmi_smb =
 		container_of(xport, struct rmi_smb_xport, xport);
-	int cur_len = (int)len;
+	size_t cur_len = len;
 
 	mutex_lock(&rmi_smb->page_mutex);
 
@@ -148,7 +148,7 @@ static int rmi_smb_write_block(struct rmi_transport_dev *xport, u16 rmiaddr,
 		/*
 		 * break into 32 bytes chunks to write get command code
 		 */
-		int block_len = min_t(int, len, SMB_MAX_COUNT);
+		int block_len = min_t(size_t, cur_len, SMB_MAX_COUNT);
 
 		retval = rmi_smb_get_command_code(xport, rmiaddr, block_len,
 						  false, &commandcode);
@@ -161,9 +161,9 @@ static int rmi_smb_write_block(struct rmi_transport_dev *xport, u16 rmiaddr,
 			goto exit;
 
 		/* prepare to write next block of bytes */
-		cur_len -= SMB_MAX_COUNT;
-		databuff += SMB_MAX_COUNT;
-		rmiaddr += SMB_MAX_COUNT;
+		cur_len -= block_len;
+		databuff += block_len;
+		rmiaddr += block_len;
 	}
 exit:
 	mutex_unlock(&rmi_smb->page_mutex);
-- 
2.55.0.571.g244d577d93-goog


-- 
Dmitry