Re: [PATCH] fdc: check null block pointer before blk_pwrite

[Li Qiang <liq3ea@gmail.com>]

P J P <ppandit@redhat.com> 于2020年8月27日周四 下午7:41写道：
>
> From: Prasad J Pandit <pjp@fedoraproject.org>
>
> While transferring data via fdctrl_write_data(), check that
> current drive does not have a null block pointer. Avoid
> null pointer dereference.
>
>  -> https://ruhr-uni-bochum.sciebo.de/s/NNWP2GfwzYKeKwE?path=%2Ffdc_nullptr1
>     ==1658854==Hint: address points to the zero page.
>     #0 blk_inc_in_flight block/block-backend.c:1327
>     #1 blk_prw block/block-backend.c:1299
>     #2 blk_pwrite block/block-backend.c:1464
>     #3 fdctrl_write_data hw/block/fdc.c:2418
>     #4 fdctrl_write hw/block/fdc.c:962
>     #5 portio_write ioport.c:205
>     #6 memory_region_write_accessor memory.c:483
>     #7 access_with_adjusted_size memory.c:544
>     #8 memory_region_dispatch_write memory.c:1476
>
> Reported-by: Ruhr-University <bugs-syssec@rub.de>
> Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
> ---
>  hw/block/fdc.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/hw/block/fdc.c b/hw/block/fdc.c
> index e9ed3eef45..dedadac68a 100644
> --- a/hw/block/fdc.c
> +++ b/hw/block/fdc.c
> @@ -2419,7 +2419,8 @@ static void fdctrl_write_data(FDCtrl *fdctrl, uint32_t value)
>          if (pos == FD_SECTOR_LEN - 1 ||
>              fdctrl->data_pos == fdctrl->data_len) {
>              cur_drv = get_cur_drv(fdctrl);
> -            if (blk_pwrite(cur_drv->blk, fd_offset(cur_drv), fdctrl->fifo,
> +            if (cur_drv->blk
> +                && blk_pwrite(cur_drv->blk, fd_offset(cur_drv), fdctrl->fifo,
>                             BDRV_SECTOR_SIZE, 0) < 0) {
>                  FLOPPY_DPRINTF("error writing sector %d\n",
>                                 fd_sector(cur_drv));
> --

Hello Prasad,

There are several pattern to address this NULL check in fdc.c.
I think it is better to consider this as an error. Just like the check
in 'fdctrl_format_sector':

if (cur_drv->blk == NULL ||
    blk_pwrite(cur_drv->blk, fd_offset(cur_drv), fdctrl->fifo,
    BDRV_SECTOR_SIZE, 0) < 0) {
    FLOPPY_DPRINTF("error formatting sector %d\n", fd_sector(cur_drv));
    fdctrl_stop_transfer(fdctrl, FD_SR0_ABNTERM | FD_SR0_SEEK, 0x00, 0x00);
} else {

Also there seems exists the same issue in  'fdctrl_read_data'

Thanks,
Li Qiang

> 2.26.2
>
>