comedi: Fix use of uninitialized data in insn_rw_emulate_bits()

For Comedi `INSN_READ` and `INSN_WRITE` instructions on "digital"
subdevices (subdevice types `COMEDI_SUBD_DI`, `COMEDI_SUBD_DO`, and
`COMEDI_SUBD_DIO`), it is common for the subdevice driver not to have
`insn_read` and `insn_write` handler functions, but to have an
`insn_bits` handler function for handling Comedi `INSN_BITS`
instructions.  In that case, the subdevice's `insn_read` and/or
`insn_write` function handler pointers are set to point to the
`insn_rw_emulate_bits()` function by `__comedi_device_postconfig()`.

For `INSN_WRITE`, `insn_rw_emulate_bits()` currently assumes that the
supplied `data[0]` value is a valid copy from user memory.  It will at
least exist because `do_insnlist_ioctl()` and `do_insn_ioctl()` in
"comedi_fops.c" ensure at lease `MIN_SAMPLES` (16) elements are
allocated.  However, if `insn->n` is 0 (which is allowable for
`INSN_READ` and `INSN_WRITE` instructions, then `data[0]` may contain
uninitialized data, and certainly contains invalid data, possibly from a
different instruction in the array of instructions handled by
`do_insnlist_ioctl()`.  This will result in an incorrect value being
written to the digital output channel (or to the digital input/output
channel if configured as an output), and may be reflected in the
internal saved state of the channel.

Fix it by returning 0 early if `insn->n` is 0, before reaching the code
that accesses `data[0]`.  Previously, the function always returned 1 on
success, but it is supposed to be the number of data samples actually
read or written up to `insn->n`, which is 0 in this case.

Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=cb96ec476fb4914445c9
Fixes: ed9eccbe8970 ("Staging: add comedi core")
Cc: [email protected] # 5.13+
Signed-off-by: Ian Abbott <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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