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Subject[PATCH v2] cdc-acm: prevent infinite loop when parsing CDC headers.
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.

- 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 <>
Signed-off-by: Quentin Casasnovas <>
CC: Sergei Shtylyov <>
CC: Oliver Neukum <>
CC: Adam Lee <>
CC: <>
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 */

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