[PATCH] hw/ide/core: Fix possible crash via NULL pointer in ide_cancel_dma_sync()

[Thomas Huth <thuth@redhat.com>]

From: Thomas Huth <thuth@redhat.com>

ide_cancel_dma_sync() is called with a "IDEState *s" for one of the
two IDE drives on a bus (primary or secondary drive) to cancel all
pending DMA transfers on the drive. The code then checks
s->bus->dma->aiocb to see whether there is any IO in flight on the
*bus* and then calls blk_drain(s->blk) to wait for its completion.
However, s->bus->dma->aiocb might belong to the other drive on the
bus, and if there is no disk attached to the current drive, s->blk
is NULL. Since blk_drain() does not check its parameter for a NULL
pointer, QEMU can crash in such a case.

Fix the problem by checking s->blk to be a valid pointer before
calling blk_drain() in this function.

Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/905
Reported-by: Alexander Bulekov <alxndr@bu.edu>
Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/4052
Reported-by: dong ling
Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 hw/ide/core.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/hw/ide/core.c b/hw/ide/core.c
index f78b00220b8..49848c8e6bd 100644
--- a/hw/ide/core.c
+++ b/hw/ide/core.c
@@ -741,8 +741,11 @@ void ide_cancel_dma_sync(IDEState *s)
      * In the future we'll be able to safely cancel the I/O if the
      * whole DMA operation will be submitted to disk with a single
      * aio operation with preadv/pwritev.
+     *
+     * Note: s->bus->dma->aiocb might belong to the adjacent IDEState,
+     * so we have to check s->blk for not being NULL, too.
      */
-    if (s->bus->dma->aiocb) {
+    if (s->bus->dma->aiocb && s->blk) {
         trace_ide_cancel_dma_sync_remaining();
         blk_drain(s->blk);
         assert(s->bus->dma->aiocb == NULL);
-- 
2.55.0