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Re: [Qemu-block] [Qemu-devel] [PATCH] ide: check BlockBackend object in


From: John Snow
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH] ide: check BlockBackend object in ide_cancel_dma_sync
Date: Wed, 19 Jul 2017 14:49:51 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1


On 07/14/2017 06:00 AM, P J P wrote:
> From: Prasad J Pandit <address@hidden>
> 
> When cancelling pending DMA requests in ide_cancel_dma_sync,
> the s->blk object could be null, leading to a null dereference.
> Add check to avoid it.
> 
> Reported-by: Chensongnian <address@hidden>
> Signed-off-by: Prasad J Pandit <address@hidden>
> ---
>  hw/ide/core.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/ide/core.c b/hw/ide/core.c
> index 0b48b64..04474b3 100644
> --- a/hw/ide/core.c
> +++ b/hw/ide/core.c
> @@ -681,8 +681,10 @@ void ide_cancel_dma_sync(IDEState *s)
>  #ifdef DEBUG_IDE
>          printf("%s: draining all remaining requests", __func__);
>  #endif
> -        blk_drain(s->blk);
> -        assert(s->bus->dma->aiocb == NULL);
> +        if (s->blk) {
> +            blk_drain(s->blk);
> +            assert(s->bus->dma->aiocb == NULL);
> +        }
>      }
>  }
>  
> 

I guess this occurs through

ide_exec_cmd
  cmd_device_reset
    ide_cancel_dma_sync

though if s->blk does not exist, we should usually not be able to
address this device with a reset command as such. (core.c:2021) -- but
this is only for secondary devices. I guess we don't guard against
nonexistent primary devices......?

Further, how do we have s->bus->dma->aiocb if there's no blk device?
What DMA request did we accept...?

Can you please submit a stack that illustrates the code path followed so
this fix can be properly verified and tested?

Thanks,
--John



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