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

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;
  }
---