media: cxusb: no longer judge rbuf when the write fails

syzbot reported a uninit-value in cxusb_i2c_xfer. [1]

Only when the write operation of usb_bulk_msg() in dvb_usb_generic_rw()
succeeds and rlen is greater than 0, the read operation of usb_bulk_msg()
will be executed to read rlen bytes of data from the dvb device into the
rbuf.

In this case, although rlen is 1, the write operation failed which resulted
in the dvb read operation not being executed, and ultimately variable i was
not initialized.

[1]
BUG: KMSAN: uninit-value in cxusb_gpio_tuner drivers/media/usb/dvb-usb/cxusb.c:124 [inline]
BUG: KMSAN: uninit-value in cxusb_i2c_xfer+0x153a/0x1a60 drivers/media/usb/dvb-usb/cxusb.c:196
cxusb_gpio_tuner drivers/media/usb/dvb-usb/cxusb.c:124 [inline]
cxusb_i2c_xfer+0x153a/0x1a60 drivers/media/usb/dvb-usb/cxusb.c:196
__i2c_transfer+0xe25/0x3150 drivers/i2c/i2c-core-base.c:-1
i2c_transfer+0x317/0x4a0 drivers/i2c/i2c-core-base.c:2315
i2c_transfer_buffer_flags+0x125/0x1e0 drivers/i2c/i2c-core-base.c:2343
i2c_master_send include/linux/i2c.h:109 [inline]
i2cdev_write+0x210/0x280 drivers/i2c/i2c-dev.c:183
do_loop_readv_writev fs/read_write.c:848 [inline]
vfs_writev+0x963/0x14e0 fs/read_write.c:1057
do_writev+0x247/0x5c0 fs/read_write.c:1101
__do_sys_writev fs/read_write.c:1169 [inline]
__se_sys_writev fs/read_write.c:1166 [inline]
__x64_sys_writev+0x98/0xe0 fs/read_write.c:1166
x64_sys_call+0x2229/0x3c80 arch/x86/include/generated/asm/syscalls_64.h:21
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xcd/0x1e0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Reported-by: [e-mail]
Closes: https://syzkaller.appspot.com/bug?extid=526bd95c0ec629993bf3
Tested-by: [e-mail]
Fixes: 22c6d93a7310 ("[PATCH] dvb: usb: support Medion hybrid USB2.0 DVB-T/analogue box")
Cc: [e-mail]
Signed-off-by: Edward Adam Davis <[e-mail]>
Signed-off-by: Hans Verkuil <[e-mail]>

drivers/media/usb/dvb-usb/cxusb.c

···
 
 	o[0] = GPIO_TUNER;
 	o[1] = onoff;
-	cxusb_ctrl_msg(d, CMD_GPIO_WRITE, o, 2, &i, 1);
 
-	if (i != 0x01)
+	if (!cxusb_ctrl_msg(d, CMD_GPIO_WRITE, o, 2, &i, 1) && i != 0x01)
 		dev_info(&d->udev->dev, "gpio_write failed.\n");
 
 	st->gpio_write_state[GPIO_TUNER] = onoff;