linux-kernel - [PATCH] comedi: Fail COMEDI_INSNLIST ioctl if n

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Message-ID: <20250704120405.83028-1-abbotti@mev.co.uk>
Date: Fri,  4 Jul 2025 13:04:05 +0100
From: Ian Abbott <abbotti@....co.uk>
To: linux-kernel@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Ian Abbott <abbotti@....co.uk>,
	H Hartley Sweeten <hsweeten@...ionengravers.com>,
	syzbot+d6995b62e5ac7d79557a@...kaller.appspotmail.com,
	stable@...r.kernel.org
Subject: [PATCH] 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@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=d6995b62e5ac7d79557a
Fixes: ed9eccbe8970 ("Staging: add comedi core")
Tested-by: Ian Abbott <abbotti@....co.uk>
Cc: <stable@...r.kernel.org> # 5.13+
Signed-off-by: Ian Abbott <abbotti@....co.uk>
---
Patch does not apply cleanly to longterm kernels 5.4.x and 5.10.x.
---
 drivers/comedi/comedi_fops.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/drivers/comedi/comedi_fops.c b/drivers/comedi/comedi_fops.c
index 3383a7ce27ff..962fb9b18a52 100644
--- a/drivers/comedi/comedi_fops.c
+++ b/drivers/comedi/comedi_fops.c
@@ -1589,6 +1589,16 @@ static int do_insnlist_ioctl(struct comedi_device *dev,
 	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;
-- 
2.47.2