comedi: Fix use of uninitialized memory in do_insn_ioctl() and do_insnlist_ioctl()

commit 3cd212e upstream.

syzbot reports a KMSAN kernel-infoleak in `do_insn_ioctl()`.  A kernel
buffer is allocated to hold `insn->n` samples (each of which is an
`unsigned int`).  For some instruction types, `insn->n` samples are
copied back to user-space, unless an error code is being returned.  The
problem is that not all the instruction handlers that need to return
data to userspace fill in the whole `insn->n` samples, so that there is
an information leak.  There is a similar syzbot report for
`do_insnlist_ioctl()`, although it does not have a reproducer for it at
the time of writing.

One culprit is `insn_rw_emulate_bits()` which is used as the handler for
`INSN_READ` or `INSN_WRITE` instructions for subdevices that do not have
a specific handler for that instruction, but do have an `INSN_BITS`
handler.  For `INSN_READ` it only fills in at most 1 sample, so if
`insn->n` is greater than 1, the remaining `insn->n - 1` samples copied
to userspace will be uninitialized kernel data.

Another culprit is `vm80xx_ai_insn_read()` in the "vm80xx" driver.  It
never returns an error, even if it fails to fill the buffer.

Fix it in `do_insn_ioctl()` and `do_insnlist_ioctl()` by making sure
that uninitialized parts of the allocated buffer are zeroed before
handling each instruction.

Thanks to Arnaud Lecomte for their fix to `do_insn_ioctl()`.  That fix
replaced the call to `kmalloc_array()` with `kcalloc()`, but it is not
always necessary to clear the whole buffer.

Fixes: ed9eccb ("Staging: add comedi core")
Reported-by: syzbot+a5e45f768aab5892da5d@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=a5e45f768aab5892da5d
Reported-by: syzbot+fb4362a104d45ab09cf9@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=fb4362a104d45ab09cf9
Cc: stable <stable@kernel.org> # 5.13+
Cc: Arnaud Lecomte <contact@arnaud-lcm.com>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20250725125324.80276-1-abbotti@mev.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: ian-abbott

drivers/comedi/comedi_fops.c

@@ -1587,6 +1587,9 @@ static int do_insnlist_ioctl(struct comedi_device *dev,
 				memset(&data[n], 0, (MIN_SAMPLES - n) *
 						    sizeof(unsigned int));
 			}
+		} else {
+			memset(data, 0, max_t(unsigned int, n, MIN_SAMPLES) *
+					sizeof(unsigned int));
 		}
 		ret = parse_insn(dev, insns + i, data, file);
 		if (ret < 0)
@@ -1670,6 +1673,8 @@ static int do_insn_ioctl(struct comedi_device *dev,
 			memset(&data[insn->n], 0,
 			       (MIN_SAMPLES - insn->n) * sizeof(unsigned int));
 		}
+	} else {
+		memset(data, 0, n_data * sizeof(unsigned int));
 	}
 	ret = parse_insn(dev, insn, data, file);
 	if (ret < 0)