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

On 21/07/2026 12.55, Philippe Mathieu-Daudé wrote:
> On 21/7/26 12:21, Thomas Huth wrote:
>> 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?
> 
> I dunno I'm kind of lost. Can't blk_bs() handle NULL blk?
> 
> -- >8 --
> diff --git a/block/block-backend.c b/block/block-backend.c
> index 37ba7e9fc40..b5d84c9789a 100644
> --- a/block/block-backend.c
> +++ b/block/block-backend.c
> @@ -760,7 +760,7 @@ BlockBackend *blk_by_name(const char *name)
>   BlockDriverState *blk_bs(BlockBackend *blk)
>   {
>       IO_CODE();
> -    return blk->root ? blk->root->bs : NULL;
> +    return (blk && blk->root) ? blk->root->bs : NULL;
>   }
That would maybe help with the NULL pointer crash, but it would not prevent 
the assert(s->bus->dma->aiocb == NULL) from triggering.

Simply draining both drives seems to be the only solution that I see right 
now, unless we want to remove the assert() statement?

  Thomas