[PATCH 3.4 168/172] cdc-acm: prevent infinite loop when parsing CDC headers.

From: lizf
Date: Tue Jun 16 2015 - 04:54:52 EST

From: Quentin Casasnovas <quentin.casasnovas@xxxxxxxxxx>

3.4.108-rc1 review patch. If anyone has any objections, please let me know.


commit 0d3bba0287d4e284c3ec7d3397e81eec920d5e7e upstream.

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@xxxxxxxxxx>
Signed-off-by: Quentin Casasnovas <quentin.casasnovas@xxxxxxxxxx>
CC: Sergei Shtylyov <sergei.shtylyov@xxxxxxxxxxxxxxxxxx>
CC: Oliver Neukum <oneukum@xxxxxxx>
CC: Adam Lee <adam8157@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Zefan Li <lizefan@xxxxxxxxxx>
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 57877a9..25bc1d6 100644
--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -1044,11 +1044,16 @@ 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 */

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