[PATCH v2] fdc: check null block pointer before r/w data transfer

[P J P <ppandit@redhat.com>]

From: Prasad J Pandit <pjp@fedoraproject.org>

While transferring data via fdctrl_read/write_data() routines,
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 | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

Update v2: treat NULL blk pointer as an error
  -> https://lists.nongnu.org/archive/html/qemu-devel/2020-09/msg06642.html

diff --git a/hw/block/fdc.c b/hw/block/fdc.c
index 224bac504f..bf968dc66f 100644
--- a/hw/block/fdc.c
+++ b/hw/block/fdc.c
@@ -1923,7 +1923,8 @@ static uint32_t fdctrl_read_data(FDCtrl *fdctrl)
                                    fd_sector(cur_drv));
                     return 0;
                 }
-            if (blk_pread(cur_drv->blk, fd_offset(cur_drv), fdctrl->fifo,
+            if (!cur_drv->blk
+                || blk_pread(cur_drv->blk, fd_offset(cur_drv), fdctrl->fifo,
                           BDRV_SECTOR_SIZE)
                 < 0) {
                 FLOPPY_DPRINTF("error getting sector %d\n",
@@ -2427,7 +2428,8 @@ 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));
--
2.26.2