Re: [Qemu-devel] [PATCH v2 2/9] ccid-card-passthru: Replace never trigger if statement by an assertion

[Marc-André Lureau]

Hi

On Thu, Feb 14, 2019 at 9:20 PM Philippe Mathieu-Daudé
<address@hidden> wrote:
>
> The right side of the comparison is the return value of can_read():
> VSCARD_IN_SIZE - card->vscard_in_pos.
> Since the 'size' argument of chardev::read() is bound to
> what chardev::can_read() returns, this condition can never happen.

I think so too, because vscard_in_pos is unchanged between the 2
callbacks (or set to 0 in break event).

>
> Add an assertion, which will always fail if card->vscard_in_pos >=
> VSCARD_IN_SIZE), since size > 0.

If "size > VSCARD_IN_SIZE - card->vscard_in_pos" this is a chardev
bug. But which backend does that?

Iow, did we ever reach the "no room for data" error?

>
> This is a quick fix for CVE-2018-18438 "Integer overflow in
> ccid_card_vscard_read() allows memory corruption".

I have a hard time to find how that memory corruption can happen. It
would be a broken chardev (one calling qemu_chr_be_write() with a size
bigger than qemu_chr_be_can_write()). It would need to be fixed. But
which one does that?

>
> Fixes: CVE-2018-18438
> Reported-by: Arash Tohidi Chafi <address@hidden>
> Suggested-by: Paolo Bonzini <address@hidden>
> Signed-off-by: Philippe Mathieu-Daudé <address@hidden>
> ---
>  hw/usb/ccid-card-passthru.c | 14 +-------------
>  1 file changed, 1 insertion(+), 13 deletions(-)
>
> diff --git a/hw/usb/ccid-card-passthru.c b/hw/usb/ccid-card-passthru.c
> index 8bb1314f49..1676b5fc05 100644
> --- a/hw/usb/ccid-card-passthru.c
> +++ b/hw/usb/ccid-card-passthru.c
> @@ -264,24 +264,12 @@ static void 
> ccid_card_vscard_handle_message(PassthruState *card,
>      }
>  }
>
> -static void ccid_card_vscard_drop_connection(PassthruState *card)
> -{
> -    qemu_chr_fe_deinit(&card->cs, true);
> -    card->vscard_in_pos = card->vscard_in_hdr = 0;
> -}
> -
>  static void ccid_card_vscard_read(void *opaque, const uint8_t *buf, int size)
>  {
>      PassthruState *card = opaque;
>      VSCMsgHeader *hdr;
>
> -    if (card->vscard_in_pos + size > VSCARD_IN_SIZE) {
> -        error_report("no room for data: pos %u +  size %d > %" PRId64 "."
> -                     " dropping connection.",
> -                     card->vscard_in_pos, size, VSCARD_IN_SIZE);
> -        ccid_card_vscard_drop_connection(card);
> -        return;
> -    }
> +    assert(size <= VSCARD_IN_SIZE - card->vscard_in_pos);
>      assert(card->vscard_in_hdr < VSCARD_IN_SIZE);
>      memcpy(card->vscard_in_data + card->vscard_in_pos, buf, size);
>      card->vscard_in_pos += size;
> --
> 2.20.1
>