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.

v2:
 - Use 12-digits sha1 to reference the offending commit.
 - Do not break from the loop and try next byte instead.
 - Move the assignment outside the 'if'.
 - Add a debug print.

Fixes: 7e860a6e7aa6 ("cdc-acm: add sanity checks")
Signed-off-by: Phil Turnbull <phil.turnb...@oracle.com>
Signed-off-by: Quentin Casasnovas <quentin.casasno...@oracle.com>
CC: Sergei Shtylyov <sergei.shtyl...@cogentembedded.com>
CC: Oliver Neukum <oneu...@suse.de>
CC: Adam Lee <adam8...@gmail.com>
CC: <sta...@vger.kernel.org>
---
 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 6836177..220c0fd 100644
--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -1133,11 +1133,16 @@ 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 */
-- 
2.0.5

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