[PATCH 2/2] mailbox: ti-msgmgr: Read exact number of trailing message bytes

[Beleswar Padhi]

ti_msgmgr_send_data() copies the message to the data registers in u32
units. This has been harmless so far because the clients (TI-SCI)
always passed its preallocated message buffer, which is sized to the
maximum message size (60 or 64 bytes, a multiple of 4), so the extra
bytes read were still within the buffer. But, with a later TI-SCI
update, the message size can be varying now and not guaranteed to be a
multiple of 4, which would trigger KASAN out-of-bounds reports.

Therefore, Build the trailing word by reading one byte at a time, only
up to the message length. The value written to the register is
unchanged.

Signed-off-by: Beleswar Padhi <b-padhi@xxxxxx>
---
 drivers/mailbox/ti-msgmgr.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/drivers/mailbox/ti-msgmgr.c b/drivers/mailbox/ti-msgmgr.c
index 425d5f9d9d0e3..44457a896510f 100644
--- a/drivers/mailbox/ti-msgmgr.c
+++ b/drivers/mailbox/ti-msgmgr.c
@@ -425,10 +425,18 @@ static int ti_msgmgr_send_data(struct mbox_chan *chan, void *data)
 
 	trail_bytes = message->len % sizeof(u32);
 	if (trail_bytes) {
-		u32 data_trail = *word_data;
-
-		/* Ensure all unused data is 0 */
-		data_trail &= 0xFFFFFFFF >> (8 * (sizeof(u32) - trail_bytes));
+		/*
+		 * Read the trailing bytes one at a time instead of as a full
+		 * u32, as the message buffer may end right after them and a
+		 * u32 read would go past the end of it. This also leaves all
+		 * unused data as 0.
+		 */
+		u8 *byte_data = (u8 *)word_data;
+		u32 data_trail = 0;
+		int i;
+
+		for (i = 0; i < trail_bytes; i++)
+			data_trail |= byte_data[i] << (8 * i);
 		writel(data_trail, data_reg);
 		data_reg += sizeof(u32);
 	}
-- 
2.34.1