media: dvb-frontends: dib3000mb: fix uninit-value in dib3000_write_reg

Syzbot reports [1] an uninitialized value issue found by KMSAN in
dib3000_read_reg().

Local u8 rb[2] is used in i2c_transfer() as a read buffer; in case
that call fails, the buffer may end up with some undefined values.

Since no elaborate error handling is expected in dib3000_write_reg(),
simply zero out rb buffer to mitigate the problem.

[1] Syzkaller report
dvb-usb: bulk message failed: -22 (6/0)
=====================================================
BUG: KMSAN: uninit-value in dib3000mb_attach+0x2d8/0x3c0 drivers/media/dvb-frontends/dib3000mb.c:758
 dib3000mb_attach+0x2d8/0x3c0 drivers/media/dvb-frontends/dib3000mb.c:758
 dibusb_dib3000mb_frontend_attach+0x155/0x2f0 drivers/media/usb/dvb-usb/dibusb-mb.c:31
 dvb_usb_adapter_frontend_init+0xed/0x9a0 drivers/media/usb/dvb-usb/dvb-usb-dvb.c:290
 dvb_usb_adapter_init drivers/media/usb/dvb-usb/dvb-usb-init.c:90 [inline]
 dvb_usb_init drivers/media/usb/dvb-usb/dvb-usb-init.c:186 [inline]
 dvb_usb_device_init+0x25a8/0x3760 drivers/media/usb/dvb-usb/dvb-usb-init.c:310
 dibusb_probe+0x46/0x250 drivers/media/usb/dvb-usb/dibusb-mb.c:110
...
Local variable rb created at:
 dib3000_read_reg+0x86/0x4e0 drivers/media/dvb-frontends/dib3000mb.c:54
 dib3000mb_attach+0x123/0x3c0 drivers/media/dvb-frontends/dib3000mb.c:758
...

Fixes: 74340b0 ("V4L/DVB (4457): Remove dib3000-common-module")
Reported-by: [email protected]
Signed-off-by: Nikita Zhandarovich <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Mauro Carvalho Chehab <[email protected]>

Author: Nikita Zhandarovich

drivers/media/dvb-frontends/dib3000mb.c

@@ -51,7 +51,7 @@ MODULE_PARM_DESC(debug, "set debugging level (1=info,2=xfer,4=setfe,8=getfe (|-a
 static int dib3000_read_reg(struct dib3000_state *state, u16 reg)
 {
 	u8 wb[] = { ((reg >> 8) | 0x80) & 0xff, reg & 0xff };
-	u8 rb[2];
+	u8 rb[2] = {};
 	struct i2c_msg msg[] = {
 		{ .addr = state->config.demod_address, .flags = 0,        .buf = wb, .len = 2 },
 		{ .addr = state->config.demod_address, .flags = I2C_M_RD, .buf = rb, .len = 2 },