[PATCH 4.9 17/22] comedi: vmk80xx: fix transfer-buffer overflows

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Johan Hovold <johan@kernel.org>

commit a23461c47482fc232ffc9b819539d1f837adf2b1 upstream.

The driver uses endpoint-sized USB transfer buffers but up until
recently had no sanity checks on the sizes.

Commit e1f13c879a7c ("staging: comedi: check validity of wMaxPacketSize
of usb endpoints found") inadvertently fixed NULL-pointer dereferences
when accessing the transfer buffers in case a malicious device has a
zero wMaxPacketSize.

Make sure to allocate buffers large enough to handle also the other
accesses that are done without a size check (e.g. byte 18 in
vmk80xx_cnt_insn_read() for the VMK8061_MODEL) to avoid writing beyond
the buffers, for example, when doing descriptor fuzzing.

The original driver was for a low-speed device with 8-byte buffers.
Support was later added for a device that uses bulk transfers and is
presumably a full-speed device with a maximum 64-byte wMaxPacketSize.

Fixes: 985cafccbf9b ("Staging: Comedi: vmk80xx: Add k8061 support")
Cc: stable@vger.kernel.org      # 2.6.31
Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20211025114532.4599-4-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/staging/comedi/drivers/vmk80xx.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/drivers/staging/comedi/drivers/vmk80xx.c
+++ b/drivers/staging/comedi/drivers/vmk80xx.c
@@ -99,6 +99,8 @@ enum {
 #define IC3_VERSION		BIT(0)
 #define IC6_VERSION		BIT(1)
 
+#define MIN_BUF_SIZE		64
+
 enum vmk80xx_model {
 	VMK8055_MODEL,
 	VMK8061_MODEL
@@ -687,12 +689,12 @@ static int vmk80xx_alloc_usb_buffers(str
 	struct vmk80xx_private *devpriv = dev->private;
 	size_t size;
 
-	size = usb_endpoint_maxp(devpriv->ep_rx);
+	size = max(usb_endpoint_maxp(devpriv->ep_rx), MIN_BUF_SIZE);
 	devpriv->usb_rx_buf = kzalloc(size, GFP_KERNEL);
 	if (!devpriv->usb_rx_buf)
 		return -ENOMEM;
 
-	size = usb_endpoint_maxp(devpriv->ep_tx);
+	size = max(usb_endpoint_maxp(devpriv->ep_rx), MIN_BUF_SIZE);
 	devpriv->usb_tx_buf = kzalloc(size, GFP_KERNEL);
 	if (!devpriv->usb_tx_buf)
 		return -ENOMEM;