[PATCH] hw/ide: check null block pointer before blk_drain

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

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

While cancelling an i/o operation via ide_cancel_dma_sync(),
check for null block pointer before calling blk_drain(). Avoid
null pointer dereference.

 -> https://ruhr-uni-bochum.sciebo.de/s/NNWP2GfwzYKeKwE?path=%2Fide_nullptr1
    ==1803100==Hint: address points to the zero page.
    #0 blk_bs ../block/block-backend.c:714
    #1 blk_drain ../block/block-backend.c:1715
    #2 ide_cancel_dma_sync ../hw/ide/core.c:723
    #3 bmdma_cmd_writeb ../hw/ide/pci.c:298
    #4 bmdma_write ../hw/ide/piix.c:75
    #5 memory_region_write_accessor ../softmmu/memory.c:483
    #6 access_with_adjusted_size ../softmmu/memory.c:544
    #7 memory_region_dispatch_write ../softmmu/memory.c:1465
    #8 flatview_write_continue ../exec.c:3176
    ...

Reported-by: Ruhr-University <bugs-syssec@rub.de>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
---
 hw/ide/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/ide/core.c b/hw/ide/core.c
index d997a78e47..038af1cd6b 100644
--- a/hw/ide/core.c
+++ b/hw/ide/core.c
@@ -718,7 +718,7 @@ void ide_cancel_dma_sync(IDEState *s)
      * whole DMA operation will be submitted to disk with a single
      * aio operation with preadv/pwritev.
      */
-    if (s->bus->dma->aiocb) {
+    if (s->blk && s->bus->dma->aiocb) {
         trace_ide_cancel_dma_sync_remaining();
         blk_drain(s->blk);
         assert(s->bus->dma->aiocb == NULL);
-- 
2.26.2