USB: ene_usb6250: Allocate enough memory for full object

The allocation of PageBuffer is 512 bytes in size, but the dereferencing
of struct ms_bootblock_idi (also size 512) happens at a calculated offset
within the allocation, which means the object could potentially extend
beyond the end of the allocation. Avoid this case by just allocating
enough space to catch any accesses beyond the end. Seen with GCC 13:

../drivers/usb/storage/ene_ub6250.c: In function 'ms_lib_process_bootblock':
../drivers/usb/storage/ene_ub6250.c:1050:44: warning: array subscript 'struct ms_bootblock_idi[0]' is partly outside array bounds of 'unsigned char[512]' [-Warray-bounds=]
 1050 |                         if (le16_to_cpu(idi->wIDIgeneralConfiguration) != MS_IDI_GENERAL_CONF)
      |                                            ^~
../include/uapi/linux/byteorder/little_endian.h:37:51: note: in definition of macro '__le16_to_cpu'
   37 | #define __le16_to_cpu(x) ((__force __u16)(__le16)(x))
      |                                                   ^
../drivers/usb/storage/ene_ub6250.c:1050:29: note: in expansion of macro 'le16_to_cpu'
 1050 |                         if (le16_to_cpu(idi->wIDIgeneralConfiguration) != MS_IDI_GENERAL_CONF)
      |                             ^~~~~~~~~~~
In file included from ../drivers/usb/storage/ene_ub6250.c:5:
In function 'kmalloc',
    inlined from 'ms_lib_process_bootblock' at ../drivers/usb/storage/ene_ub6250.c:942:15:
../include/linux/slab.h:580:24: note: at offset [256, 512] into object of size 512 allocated by 'kmalloc_trace'
  580 |                 return kmalloc_trace(
      |                        ^~~~~~~~~~~~~~
  581 |                                 kmalloc_caches[kmalloc_type(flags)][index],
      |                                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  582 |                                 flags, size);
      |                                 ~~~~~~~~~~~~

Cc: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230204183546.never.849-kees@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/storage/ene_ub6250.c

@@ -939,7 +939,7 @@ static int ms_lib_process_bootblock(struct us_data *us, u16 PhyBlock, u8 *PageDa
 	struct ms_lib_type_extdat ExtraData;
 	struct ene_ub6250_info *info = (struct ene_ub6250_info *) us->extra;
 
-	PageBuffer = kmalloc(MS_BYTES_PER_PAGE, GFP_KERNEL);
+	PageBuffer = kzalloc(MS_BYTES_PER_PAGE * 2, GFP_KERNEL);
 	if (PageBuffer == NULL)
 		return (u32)-1;