[PATCH v2] hw/ide: check null block before _cancel_dma_sync

[P J P]

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

When cancelling an i/o operation via ide_cancel_dma_sync(),
a block pointer may be null. Add check to 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 | 1 +
 hw/ide/pci.c  | 5 ++++-
 2 files changed, 5 insertions(+), 1 deletion(-)

Update v2: use an assert() call
 -> https://lists.nongnu.org/archive/html/qemu-devel/2020-08/msg08336.html

diff --git a/hw/ide/core.c b/hw/ide/core.c
index f76f7e5234..8105187f49 100644
--- a/hw/ide/core.c
+++ b/hw/ide/core.c
@@ -718,6 +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.
      */
+    assert(s->blk);
     if (s->bus->dma->aiocb) {
         trace_ide_cancel_dma_sync_remaining();
         blk_drain(s->blk);
diff --git a/hw/ide/pci.c b/hw/ide/pci.c
index b50091b615..b47e675456 100644
--- a/hw/ide/pci.c
+++ b/hw/ide/pci.c
@@ -295,7 +295,10 @@ void bmdma_cmd_writeb(BMDMAState *bm, uint32_t val)
     /* Ignore writes to SSBM if it keeps the old value */
     if ((val & BM_CMD_START) != (bm->cmd & BM_CMD_START)) {
         if (!(val & BM_CMD_START)) {
-            ide_cancel_dma_sync(idebus_active_if(bm->bus));
+            IDEState *s = idebus_active_if(bm->bus);
+            if (s->blk) {
+                ide_cancel_dma_sync(s);
+            }
             bm->status &= ~BM_STATUS_DMAING;
         } else {
             bm->cur_addr = bm->addr;
-- 
2.26.2