comedi: Fail COMEDI_INSNLIST ioctl if n_insns is too large

The handling of the `COMEDI_INSNLIST` ioctl allocates a kernel buffer to
hold the array of `struct comedi_insn`, getting the length from the
`n_insns` member of the `struct comedi_insnlist` supplied by the user.
The allocation will fail with a WARNING and a stack dump if it is too
large.

Avoid that by failing with an `-EINVAL` error if the supplied `n_insns`
value is unreasonable.

Define the limit on the `n_insns` value in the `MAX_INSNS` macro.  Set
this to the same value as `MAX_SAMPLES` (65536), which is the maximum
allowed sum of the values of the member `n` in the array of `struct
comedi_insn`, and sensible comedi instructions will have an `n` of at
least 1.

Reported-by: syzbot+d6995b62e5ac7d79557a@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=d6995b62e5ac7d79557a
Fixes: ed9eccbe8970 ("Staging: add comedi core")
Tested-by: Ian Abbott <abbotti@mev.co.uk>
Cc: stable@vger.kernel.org # 5.13+
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20250704120405.83028-1-abbotti@mev.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/comedi/comedi_fops.c b/drivers/comedi/comedi_fops.c
index 3383a7ce27ff55b79c662467c38494045bcd2d9f..962fb9b18a52a08e89c15549a8bf6a1ecd79e6c9 100644 (file)
--- a/drivers/comedi/comedi_fops.c
+++ b/drivers/comedi/comedi_fops.c
@@ -1589,6 +1589,16 @@ error:
        return i;
 }
 
+#define MAX_INSNS   MAX_SAMPLES
+static int check_insnlist_len(struct comedi_device *dev, unsigned int n_insns)
+{
+       if (n_insns > MAX_INSNS) {
+               dev_dbg(dev->class_dev, "insnlist length too large\n");
+               return -EINVAL;
+       }
+       return 0;
+}
+
 /*
  * COMEDI_INSN ioctl
  * synchronous instruction
@@ -2239,6 +2249,9 @@ static long comedi_unlocked_ioctl(struct file *file, unsigned int cmd,
                        rc = -EFAULT;
                        break;
                }
+               rc = check_insnlist_len(dev, insnlist.n_insns);
+               if (rc)
+                       break;
                insns = kcalloc(insnlist.n_insns, sizeof(*insns), GFP_KERNEL);
                if (!insns) {
                        rc = -ENOMEM;
@@ -3142,6 +3155,9 @@ static int compat_insnlist(struct file *file, unsigned long arg)
        if (copy_from_user(&insnlist32, compat_ptr(arg), sizeof(insnlist32)))
                return -EFAULT;
 
+       rc = check_insnlist_len(dev, insnlist32.n_insns);
+       if (rc)
+               return rc;
        insns = kcalloc(insnlist32.n_insns, sizeof(*insns), GFP_KERNEL);
        if (!insns)
                return -ENOMEM;