[PATCH 3/3] media: stv0900: bound DiSEqC reply length to msg[] size

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

The FIFO_BYTENBR field is 4 bits (mask 0x0f), giving a length of 0..15
but reply->msg is __u8[4] in struct dvb_diseqc_slave_reply.  A faulty or
malicious device reporting more than 4 bytes will the array and clobber
the stack.

The stb0899, tda10071, and s5h1420 drivers all properly bound the FIFO
count against sizeof(reply->msg) before the read loop, so do the same
thing in this driver.

Cc: Hans Verkuil <hverkuil@kernel.org>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Fixes: 99277b3824e4 ("V4L/DVB (10803): Add core code for ST STV0900 dual demodulator.")
Cc: stable <stable@kernel.org>
Assisted-by: gregkh_clanker_t1000
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/media/dvb-frontends/stv0900_core.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/media/dvb-frontends/stv0900_core.c b/drivers/media/dvb-frontends/stv0900_core.c
index d15c55de2723..0ca6b6d81273 100644
--- a/drivers/media/dvb-frontends/stv0900_core.c
+++ b/drivers/media/dvb-frontends/stv0900_core.c
@@ -1773,6 +1773,8 @@ static int stv0900_recv_slave_reply(struct dvb_frontend *fe,
 
 	if (stv0900_get_bits(intp, RX_END)) {
 		reply->msg_len = stv0900_get_bits(intp, FIFO_BYTENBR);
+		if (reply->msg_len > sizeof(reply->msg))
+			reply->msg_len = sizeof(reply->msg);
 
 		for (i = 0; i < reply->msg_len; i++)
 			reply->msg[i] = stv0900_read_reg(intp, DISRXDATA);
-- 
2.53.0