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

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Message-Id: <1429003543-15856-1-git-send-email-quentin.casasnovas@oracle.com>
Date:	Tue, 14 Apr 2015 11:25:43 +0200
From:	Quentin Casasnovas <quentin.casasnovas@...cle.com>
To:	lkml <linux-kernel@...r.kernel.org>
Cc:	linux-usb <linux-usb@...r.kernel.org>,
	Adam Lee <adam8157@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
	Oliver Neukum <oneukum@...e.de>,
	Phil Turnbull <phil.turnbull@...cle.com>,
	Quentin Casasnovas <quentin.casasnovas@...cle.com>
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.

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.turnbull@...cle.com>
Signed-off-by: Quentin Casasnovas <quentin.casasnovas@...cle.com>
CC: Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
CC: Oliver Neukum <oneukum@...e.de>
CC: Adam Lee <adam8157@...il.com>
CC: <stable@...r.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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