mailbox: ti-msgmgr: Read exact number of trailing message bytes

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, memcpy() only the trailing bytes into a zeroed u32 before
writing it to the register. This is at most 3 bytes, so the overhead
is negligible.

Fixes: aace66b170 ("mailbox: Introduce TI message manager driver")
Signed-off-by: Beleswar Padhi <b-padhi@ti.com>
Acked-by: Andrew Davis <afd@ti.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>

drivers/mailbox/ti-msgmgr.c

@@ -20,6 +20,7 @@
 #include <linux/platform_device.h>
 #include <linux/property.h>
 #include <linux/soc/ti/ti-msgmgr.h>
+#include <linux/string.h>
 
 #define Q_DATA_OFFSET(proxy, queue, reg)	\
 		     ((0x10000 * (proxy)) + (0x80 * (queue)) + ((reg) * 4))
@@ -425,10 +426,15 @@ 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;
+		/*
+		 * Copy only the trailing bytes instead of reading 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.
+		 */
+		u32 data_trail = 0;
 
-		/* Ensure all unused data is 0 */
-		data_trail &= 0xFFFFFFFF >> (8 * (sizeof(u32) - trail_bytes));
+		memcpy(&data_trail, word_data, trail_bytes);
 		writel(data_trail, data_reg);
 		data_reg += sizeof(u32);
 	}