linux-kernel - [PATCH 3.2 06/94] usb: gadget: f_fs: fix NULL pointer dereference when there are no strings

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Message-ID: <lsq.1407170911.532055220@decadent.org.uk>
Date:	Mon, 04 Aug 2014 17:48:31 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, "Michal Nazarewicz" <mina86@...a86.com>,
	"Felipe Balbi" <balbi@...com>
Subject: [PATCH 3.2 06/94] usb: gadget: f_fs: fix NULL pointer dereference
 when there are no strings

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

------------------

From: Michal Nazarewicz <mina86@...a86.com>

commit f0688c8b81d2ea239c3fb0b848f623b579238d99 upstream.

If the descriptors do not need any strings and user space sends empty
set of strings, the ffs->stringtabs field remains NULL.  Thus
*ffs->stringtabs in functionfs_bind leads to a NULL pointer
dereferenece.

The bug was introduced by commit [fd7c9a007f: “use usb_string_ids_n()”].

While at it, remove double initialisation of lang local variable in
that function.

ffs->strings_count does not need to be checked in any way since in
the above scenario it will remain zero and usb_string_ids_n() is
a no-operation when colled with 0 argument.

Signed-off-by: Michal Nazarewicz <mina86@...a86.com>
Signed-off-by: Felipe Balbi <balbi@...com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/usb/gadget/f_fs.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

--- a/drivers/usb/gadget/f_fs.c
+++ b/drivers/usb/gadget/f_fs.c
@@ -1376,11 +1376,13 @@ static int functionfs_bind(struct ffs_da
 	ffs->ep0req->context = ffs;
 
 	lang = ffs->stringtabs;
-	for (lang = ffs->stringtabs; *lang; ++lang) {
-		struct usb_string *str = (*lang)->strings;
-		int id = first_id;
-		for (; str->s; ++id, ++str)
-			str->id = id;
+	if (lang) {
+		for (; *lang; ++lang) {
+			struct usb_string *str = (*lang)->strings;
+			int id = first_id;
+			for (; str->s; ++id, ++str)
+				str->id = id;
+		}
 	}
 
 	ffs->gadget = cdev->gadget;

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