Hi
On Thu, Feb 14, 2019 at 9:20 PM Philippe Mathieu-Daudé
<phi...@redhat.com> 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 <tohidi.ar...@gmail.com>
> Suggested-by: Paolo Bonzini <pbonz...@redhat.com>
> Signed-off-by: Philippe Mathieu-Daudé <phi...@redhat.com>
> ---
> 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
>
