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

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Message-Id: <1403020061-20890-1-git-send-email-mina86@mina86.com>
Date:	Tue, 17 Jun 2014 17:47:41 +0200
From:	Michal Nazarewicz <mina86@...a86.com>
To:	Felipe Balbi <balbi@...com>
Cc:	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	<stable@...r.kernel.org>, Michal Nazarewicz <mina86@...a86.com>
Subject: [PATCH] usb: gadget: f_fs: fix NULL pointer dereference when there are no strings

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.

Cc: <stable@...r.kernel.org>  # v2.6.36+
Signed-off-by: Michal Nazarewicz <mina86@...a86.com>
---
 drivers/usb/gadget/f_fs.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/drivers/usb/gadget/f_fs.c b/drivers/usb/gadget/f_fs.c
index 74202d6..8598c27 100644
--- a/drivers/usb/gadget/f_fs.c
+++ b/drivers/usb/gadget/f_fs.c
@@ -1483,11 +1483,13 @@ static int functionfs_bind(struct ffs_data *ffs, struct usb_composite_dev *cdev)
 	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;
-- 
2.0.0.526.g5318336

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