[PATCH] media: cec: stm32: prevent out-of-bounds write on RX overflow

[Weigang He <geoffreyhe2@gmail.com>]

stm32_rx_done() appends each received CEC byte to rx_msg.msg[] using
rx_msg.len as the write index, incrementing it on every RXBR
(receive-byte-ready) interrupt without checking it against the buffer
size:

	cec->rx_msg.msg[cec->rx_msg.len++] = val & 0xFF;

rx_msg.msg[] is a fixed CEC_MAX_MSG_SIZE (16) byte array in struct
cec_msg, and rx_msg.len is only reset on RXACKE/RXOVR or after a
completed message (RXEND). The number of bytes received before RXEND is
decided by the remote CEC device (it sets EOM), not by the driver. A
peer that keeps sending bytes without ending the message drives RXBR
repeatedly, pushing rx_msg.len past 16 and writing peer-controlled bytes
out of bounds into the surrounding memory. This is reachable in normal
operation once the driver has probed and receiving is enabled, from the
IRQ thread, without any local privilege.

The length check in the CEC core runs on the consumer side, after the
byte has been stored, so it does not prevent the overflow. Bound the
index in the driver before the store, as the other platform CEC drivers
already do (e.g. tegra_cec), dropping the excess bytes of an overlong
frame.

Found by static analysis tool CodeQL.

Fixes: d69ae57453c8 ("[media] cec: add STM32 cec driver")
Cc: stable@vger.kernel.org
Signed-off-by: Weigang He <geoffreyhe2@gmail.com>
---
 drivers/media/cec/platform/stm32/stm32-cec.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/media/cec/platform/stm32/stm32-cec.c b/drivers/media/cec/platform/stm32/stm32-cec.c
index 1ec0cece0a5b7..8c2fc232202de 100644
--- a/drivers/media/cec/platform/stm32/stm32-cec.c
+++ b/drivers/media/cec/platform/stm32/stm32-cec.c
@@ -132,7 +132,8 @@ static void stm32_rx_done(struct stm32_cec *cec, u32 status)
 		u32 val;
 
 		regmap_read(cec->regmap, CEC_RXDR, &val);
-		cec->rx_msg.msg[cec->rx_msg.len++] = val & 0xFF;
+		if (cec->rx_msg.len < CEC_MAX_MSG_SIZE)
+			cec->rx_msg.msg[cec->rx_msg.len++] = val & 0xFF;
 	}
 
 	if (cec->irq_status & RXEND) {

base-commit: 9716c086c8e8b141d35aa61f2e96a2e83de212a7
-- 
2.43.0