si2168: Bounds check firmware

si2168: Bounds check firmware

commit 47810b4341ac9d2f558894bc5995e6fa2a1298f9 upstream.

When reading the firmware and sending commands, the length must
be bounds checked to avoid overrunning the size of the command
buffer and smashing the stack if the firmware is not in the expected
format:

si2168 11-0064: found a 'Silicon Labs Si2168-B40'
si2168 11-0064: downloading firmware from file 'dvb-demod-si2168-b40-01.fw'
si2168 11-0064: firmware download failed -95
Kernel panic - not syncing: stack-protector: Kernel stack is corrupted in: ffffffffa085708f

Add the proper check.

Reported-by: Stuart Auchterlonie <sauchter@redhat.com>
Reviewed-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Laura Abbott <labbott@fedoraproject.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>