[PATCH v2] usb: gadget: configfs: fix OOB read in ext_prop_data_show()

[Bingquan Chen <patzilla007@gmail.com>]

In ext_prop_data_store(), for unicode property types, the data buffer
is allocated via kmemdup() with size 'len', but data_len is set to
len*2+2 to account for the UTF-16 encoding and a 2-byte null
terminator, as required by the Microsoft OS Extended Properties
Descriptor specification (dwPropertyDataLength must include the
terminator).

However, the null terminator is never actually stored in the data
buffer. When ext_prop_data_show() reads the data back, it computes the
read length as data_len >> 1 = len+1, then does memcpy(page, data,
len+1), reading 1 byte past the allocated buffer. This is a
slab-out-of-bounds read that leaks 1 byte of adjacent heap data to
userspace via configfs.

KASAN report (5.10.252):

  BUG: KASAN: slab-out-of-bounds in ext_prop_data_show+0x4a/0x60
  Read of size 9 at addr ffff888005546008 by task poc/62

  Allocated by task 62:
   kmemdup+0x17/0x40
   ext_prop_data_store+0x52/0x130
   configfs_write_file+0x168/0x200

  The buggy address belongs to the object at ffff888005546008
   which belongs to the cache kmalloc-8 of size 8

Fix by allocating len+2 bytes and explicitly zero-terminating with a
full 2-byte UTF-16 null terminator. This ensures the buffer fully
matches the dwPropertyDataLength semantics (len*2+2) while eliminating
the OOB read.

Fixes: 7419485f197c ("usb: gadget: configfs: OS Extended Properties descriptors support")
Cc: stable@vger.kernel.org
Signed-off-by: Bingquan Chen <patzilla007@gmail.com>
---
 drivers/usb/gadget/configfs.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/gadget/configfs.c b/drivers/usb/gadget/configfs.c
index 183a25f65ac8..b2c3d4e5f6a7 100644
--- a/drivers/usb/gadget/configfs.c
+++ b/drivers/usb/gadget/configfs.c
@@ -1352,8 +1352,12 @@ static ssize_t ext_prop_data_store(struct config_item *item,

 	if (page[len - 1] == '\n' || page[len - 1] == '\0')
 		--len;
-	new_data = kmemdup(page, len, GFP_KERNEL);
+	new_data = kmalloc(len + 2, GFP_KERNEL);
 	if (!new_data)
 		return -ENOMEM;
+	memcpy(new_data, page, len);
+	new_data[len]     = '\0';
+	new_data[len + 1] = '\0';

 	if (desc->opts_mutex)
--
2.43.0