[PATCH] usb: gadget: configfs: fix 1-byte 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 inflated
to len*2+2 to account for the UTF-16 encoding and a 2-byte null
terminator. The null terminator is not 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+1 bytes and null-terminating the buffer, so the
extra byte read in show() returns a known-zero byte instead of
adjacent slab data.

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 | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/gadget/configfs.c b/drivers/usb/gadget/configfs.c
index 183a25f65ac8..a1b2c3d4e5f6 100644
--- a/drivers/usb/gadget/configfs.c
+++ b/drivers/usb/gadget/configfs.c
@@ -1352,8 +1352,11 @@ 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 + 1, GFP_KERNEL);
 	if (!new_data)
 		return -ENOMEM;
+	memcpy(new_data, page, len);
+	new_data[len] = '\0';

 	if (desc->opts_mutex)
--
2.43.0