linux-hardening - Re: [PATCH] USB: serial: Fix heap overflow in WHITEHEAT_GET_DTR

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Date:   Wed, 20 Apr 2022 09:54:19 +0200
From:   Johan Hovold <johan@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     kernel test robot <lkp@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, stable@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] USB: serial: Fix heap overflow in WHITEHEAT_GET_DTR_RTS

On Mon, Apr 18, 2022 at 09:17:42PM -0700, Kees Cook wrote:
> This looks like it's harmless, as both the source and the destinations are
> currently the same allocation size (4 bytes) and don't use their padding,
> but if anything were to ever be added after the "mcr" member in "struct
> whiteheat_private", it would be overwritten. The structs both have a
> single u8 "mcr" member, but are 4 bytes in padded size. The memcpy()
> destination was explicitly targeting the u8 member (size 1) with the
> length of the whole structure (size 4), triggering the memcpy buffer
> overflow warning:

Ehh... No. The size of a structure with a single u8 is 1, not 4. There's
nothing wrong with the current code even if the use of memcpy for this
is a bit odd.

> In file included from include/linux/string.h:253,
>                  from include/linux/bitmap.h:11,
>                  from include/linux/cpumask.h:12,
>                  from include/linux/smp.h:13,
>                  from include/linux/lockdep.h:14,
>                  from include/linux/spinlock.h:62,
>                  from include/linux/mmzone.h:8,
>                  from include/linux/gfp.h:6,
>                  from include/linux/slab.h:15,
>                  from drivers/usb/serial/whiteheat.c:17:
> In function 'fortify_memcpy_chk',
>     inlined from 'firm_send_command' at drivers/usb/serial/whiteheat.c:587:4:
> include/linux/fortify-string.h:328:25: warning: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Wattribute-warning]
>   328 |                         __write_overflow_field(p_size_field, size);
>       |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

So something is confused here.
 
> Expand the memcpy() to the entire structure, though perhaps the correct
> solution is to mark all the USB command structures as "__packed".

Again no, why would you potentially overwrite the whole structure just to
update a single field? This is just wrong.

And the only structure that needs a __packed which doesn't have it
already is the unused struct whiteheat_dump.

> Reported-by: kernel test robot <lkp@...el.com>
> Link: https://lore.kernel.org/lkml/202204142318.vDqjjSFn-lkp@intel.com
> Cc: Johan Hovold <johan@...nel.org>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: linux-usb@...r.kernel.org
> Cc: stable@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  drivers/usb/serial/whiteheat.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/serial/whiteheat.c b/drivers/usb/serial/whiteheat.c
> index da65d14c9ed5..6e00498843fb 100644
> --- a/drivers/usb/serial/whiteheat.c
> +++ b/drivers/usb/serial/whiteheat.c
> @@ -584,7 +584,7 @@ static int firm_send_command(struct usb_serial_port *port, __u8 command,
>  		switch (command) {
>  		case WHITEHEAT_GET_DTR_RTS:
>  			info = usb_get_serial_port_data(port);
> -			memcpy(&info->mcr, command_info->result_buffer,
> +			memcpy(info, command_info->result_buffer,
>  					sizeof(struct whiteheat_dr_info));
>  				break;
>  		}

Johan