[Trusty/Utopic/Vivid][PATCH] cdc-acm: prevent infinite loop when parsing CDC headers.

[Adam Lee]

From: Quentin Casasnovas <quentin.casasnovas at oracle.com>

BugLink: https://bugs.launchpad.net/1460657

Phil and I found out a problem with commit:

  7e860a6e7aa6 ("cdc-acm: add sanity checks")

It added some sanity checks to ignore potential garbage in CDC headers but
also introduced a potential infinite loop.  This can happen at the first
loop iteration (elength = 0 in that case) if the description isn't a
DT_CS_INTERFACE or later if 'buffer[0]' is zero.

It should also be noted that the wrong length was being added to 'buffer'
in case 'buffer[1]' was not a DT_CS_INTERFACE descriptor, since elength was
assigned after that check in the loop.

A specially crafted USB device could be used to trigger this infinite loop.

Fixes: 7e860a6e7aa6 ("cdc-acm: add sanity checks")
Signed-off-by: Phil Turnbull <phil.turnbull at oracle.com>
Signed-off-by: Quentin Casasnovas <quentin.casasnovas at oracle.com>
CC: Sergei Shtylyov <sergei.shtylyov at cogentembedded.com>
CC: Oliver Neukum <oneukum at suse.de>
CC: Adam Lee <adam8157 at gmail.com>
CC: <stable at vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
(cherry picked from commit 0d3bba0287d4e284c3ec7d3397e81eec920d5e7e)
Signed-off-by: Adam Lee <adam.lee at canonical.com>
---
 drivers/usb/class/cdc-acm.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
index 82dea40..46321cd 100644
--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -985,11 +985,16 @@ static int acm_probe(struct usb_interface *intf,
 	}
 
 	while (buflen > 0) {
+		elength = buffer[0];
+		if (!elength) {
+			dev_err(&intf->dev, "skipping garbage byte\n");
+			elength = 1;
+			goto next_desc;
+		}
 		if (buffer[1] != USB_DT_CS_INTERFACE) {
 			dev_err(&intf->dev, "skipping garbage\n");
 			goto next_desc;
 		}
-		elength = buffer[0];
 
 		switch (buffer[2]) {
 		case USB_CDC_UNION_TYPE: /* we've found it */
-- 
2.1.4