Some Comedi subdevice instruction handlers are known to access
instruction data elements beyond the first `insn->n` elements in some
cases.  The `do_insn_ioctl()` and `do_insnlist_ioctl()` functions
allocate at least `MIN_SAMPLES` (16) data elements to deal with this,
but they do not initialize all of that.  For Comedi instruction codes
that write to the subdevice, the first `insn->n` data elements are
copied from user-space, but the remaining elements are left
uninitialized.  That could be a problem if the subdevice instruction
handler reads the uninitialized data.  Ensure that the first
`MIN_SAMPLES` elements are initialized before calling these instruction
handlers, filling the uncopied elements with 0.  For
`do_insnlist_ioctl()`, the same data buffer elements are used for
handling a list of instructions, so ensure the first `MIN_SAMPLES`
elements are initialized for each instruction that writes to the
subdevice.

Fixes: ed9eccbe8970 ("Staging: add comedi core")
Cc: <stable@vger.kernel.org> # 5.13+
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
---
Patch does not apply cleanly to longterm kernels 5.4.x and 5.10.x.
---
 drivers/comedi/comedi_fops.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/comedi/comedi_fops.c b/drivers/comedi/comedi_fops.c
index 3383a7ce27ff..0f8b471d5032 100644
--- a/drivers/comedi/comedi_fops.c
+++ b/drivers/comedi/comedi_fops.c
@@ -1556,21 +1556,27 @@ static int do_insnlist_ioctl(struct comedi_device *dev,
 	}
 
 	for (i = 0; i < n_insns; ++i) {
+		unsigned int n = insns[i].n;
+
 		if (insns[i].insn & INSN_MASK_WRITE) {
 			if (copy_from_user(data, insns[i].data,
-					   insns[i].n * sizeof(unsigned int))) {
+					   n * sizeof(unsigned int))) {
 				dev_dbg(dev->class_dev,
 					"copy_from_user failed\n");
 				ret = -EFAULT;
 				goto error;
 			}
+			if (n < MIN_SAMPLES) {
+				memset(&data[n], 0, (MIN_SAMPLES - n) *
+						    sizeof(unsigned int));
+			}
 		}
 		ret = parse_insn(dev, insns + i, data, file);
 		if (ret < 0)
 			goto error;
 		if (insns[i].insn & INSN_MASK_READ) {
 			if (copy_to_user(insns[i].data, data,
-					 insns[i].n * sizeof(unsigned int))) {
+					 n * sizeof(unsigned int))) {
 				dev_dbg(dev->class_dev,
 					"copy_to_user failed\n");
 				ret = -EFAULT;
@@ -1633,6 +1639,10 @@ static int do_insn_ioctl(struct comedi_device *dev,
 			ret = -EFAULT;
 			goto error;
 		}
+		if (insn->n < MIN_SAMPLES) {
+			memset(&data[insn->n], 0,
+			       (MIN_SAMPLES - insn->n) * sizeof(unsigned int));
+		}
 	}
 	ret = parse_insn(dev, insn, data, file);
 	if (ret < 0)
-- 
2.47.2