Re: [PATCH v2] hw/ide/core: Fix possible crash via NULL pointer in ide_cancel_dma_sync()

On 21/07/2026 11.58, Philippe Mathieu-Daudé wrote:
> On 21/7/26 11:51, Philippe Mathieu-Daudé wrote:
>> On 21/7/26 10:29, Mark Cave-Ayland wrote:
>>> On 21/07/2026 08:02, Thomas Huth wrote:
>>>
>>>> From: Thomas Huth <[email protected]>
>>>>
>>>> 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.
>>>>
>>>> To fix the problem, we have to check that "blk" is not NULL before
>>>> calling blk_drain(). And we have to call blk_drain() for both drives,
>>>> otherwise the assert(s->bus->dma->aiocb == NULL) statement after
>>>> the blk_drain() might trigger if the IO in flight belongs to the
>>>> the other drive.
>>>>
>>>> Resolves: https://urldefense.proofpoint.com/v2/url? 
>>>> u=https-3A__gitlab.com_qemu-2Dproject_qemu_-2D_work-5Fitems_905&d=DwIDAg&c=s883GpUCOChKOHiocYtGcg&r=c23RpsaH4D2MKyD3EPJTDa0BAxz6tV8aUJqVSoytEiY&m=BbZFP82dEZMyWzN5GnJ9XaxwmyrmYGbqWbEBNSUM_afB5JRRF3InIaZAr5UpUUh2&s=rCgSz13wVfk11Z-Uw5WTQViAQKkSvwpQqf_dJX1tiro&e=
>>>> Reported-by: Alexander Bulekov <[email protected]>
>>>> Resolves: https://urldefense.proofpoint.com/v2/url? 
>>>> u=https-3A__gitlab.com_qemu-2Dproject_qemu_-2D_work-5Fitems_4052&d=DwIDAg&c=s883GpUCOChKOHiocYtGcg&r=c23RpsaH4D2MKyD3EPJTDa0BAxz6tV8aUJqVSoytEiY&m=BbZFP82dEZMyWzN5GnJ9XaxwmyrmYGbqWbEBNSUM_afB5JRRF3InIaZAr5UpUUh2&s=B-AkqMAsQwA0LzU5ptwPJ3ZIBAA8ncf5ZbQg8GTwaME&e=
>>>> Reported-by: dong ling
>>>> Signed-off-by: Thomas Huth <[email protected]>
>>>> ---
>>>>   v2: Drain both drives
>>>>
>>>>   hw/ide/core.c | 9 ++++++++-
>>>>   1 file changed, 8 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/hw/ide/core.c b/hw/ide/core.c
>>>> index f78b00220b8..f43ab95b07b 100644
>>>> --- a/hw/ide/core.c
>>>> +++ b/hw/ide/core.c
>>>> @@ -741,10 +741,17 @@ 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 drain both drives to get it cleared.
>>>>        */
>>>>       if (s->bus->dma->aiocb) {
>>>>           trace_ide_cancel_dma_sync_remaining();
>>>> -        blk_drain(s->blk);
>>>> +        for (int i = 0; i < 2; i++) {
>>>> +            if (s->bus->ifs[i].blk) {
>>>> +                blk_drain(s->bus->ifs[i].blk);
>>>> +            }
>>>> +        }
>>>>           assert(s->bus->dma->aiocb == NULL);
>>>>       }
>>>>   }
>>>
>>> Doesn't ide_bus_active_if() do the right thing here for in-flight requests?
>>
>> bmdma_cmd_writeb() already calls ide_bus_active_if().
>>
>> Is the issue in the caller, bmdma_cmd_writeb(), instead of
>> here in ide_cancel_dma_sync()?
> 
> i.e.:
> 
> -- >8 --
> diff --git a/hw/ide/pci.c b/hw/ide/pci.c
> index 7ce1ae67ab2..e7c6af9a49e 100644
> --- a/hw/ide/pci.c
> +++ b/hw/ide/pci.c
> @@ -384,7 +384,7 @@ 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(ide_bus_active_if(bm->bus));
> +            ide_cancel_dma_sync(bmdma_active_if(bm));
>               bm->status &= ~BM_STATUS_DMAING;
>           } else {
>               bm->cur_addr = bm->addr;
No, I don't think this will fix all cases. While 
https://gitlab.com/qemu-project/qemu/-/work_items/905 has a call trace with 
bmdma_cmd_writeb() in it, 
https://gitlab.com/qemu-project/qemu/-/work_items/4052 is slightly 
different: The backtrace shows ide_bus_perform_srst() ->  ide_perform_srst() 
-> ide_cancel_dma_sync() here instead. I'd say let's play safe and always 
drain both interfaces. WDYT?

  Thomas