Re: [PATCH v7 1/6] memory: prevent dma-reentracy issues

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

Hi Alex,

Sorry for the late review, *sigh*.

On 13/3/23 09:24, Alexander Bulekov wrote:
> Add a flag to the DeviceState, when a device is engaged in PIO/MMIO/DMA.
> This flag is set/checked prior to calling a device's MemoryRegion
> handlers, and set when device code initiates DMA.  The purpose of this
> flag is to prevent two types of DMA-based reentrancy issues:
> 
> 1.) mmio -> dma -> mmio case
> 2.) bh -> dma write -> mmio case
> 
> These issues have led to problems such as stack-exhaustion and
> use-after-frees.
> 
> Summary of the problem from Peter Maydell:
> https://lore.kernel.org/qemu-devel/CAFEAcA_23vc7hE3iaM-JVA6W38LK4hJoWae5KcknhPRD5fPBZA@mail.gmail.com
> 
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/62
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/540
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/541
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/556
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/557
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/827
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1282
> 
> Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
> Acked-by: Peter Xu <peterx@redhat.com>
> ---
>   include/hw/qdev-core.h |  7 +++++++
>   softmmu/memory.c       | 17 +++++++++++++++++
>   softmmu/trace-events   |  1 +
>   3 files changed, 25 insertions(+)
> 
> diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
> index bd50ad5ee1..7623703943 100644
> --- a/include/hw/qdev-core.h
> +++ b/include/hw/qdev-core.h
> @@ -162,6 +162,10 @@ struct NamedClockList {
>       QLIST_ENTRY(NamedClockList) node;
>   };
>   
> +typedef struct {
> +    bool engaged_in_io;

Do you plan to add more fields?

> +} MemReentrancyGuard;
> +
>   /**
>    * DeviceState:
>    * @realized: Indicates whether the device has been fully constructed.
> @@ -194,6 +198,9 @@ struct DeviceState {
>       int alias_required_for_version;
>       ResettableState reset;
>       GSList *unplug_blockers;
> +
> +    /* Is the device currently in mmio/pio/dma? Used to prevent re-entrancy */
> +    MemReentrancyGuard mem_reentrancy_guard;

At this point I'm not sure anymore this is a device or MR property.

>   };
>   
>   struct DeviceListener {
> diff --git a/softmmu/memory.c b/softmmu/memory.c
> index 4699ba55ec..57bf18a257 100644
> --- a/softmmu/memory.c
> +++ b/softmmu/memory.c
> @@ -533,6 +533,7 @@ static MemTxResult access_with_adjusted_size(hwaddr addr,
>       uint64_t access_mask;
>       unsigned access_size;
>       unsigned i;
> +    DeviceState *dev = NULL;
>       MemTxResult r = MEMTX_OK;
>   
>       if (!access_size_min) {
> @@ -542,6 +543,19 @@ static MemTxResult access_with_adjusted_size(hwaddr addr,
>           access_size_max = 4;
>       }
>   
> +    /* Do not allow more than one simultanous access to a device's IO Regions */

Typo "simultaneous".

1/ access_with_adjusted_size() is complex enough and we are having hard
    time getting it right. I'd prefer we don't intermix size adjustment
    and re-entrancy check in the same function. This check could belong
    to the callers.

2/ I'm not keen on calling QOM object_dynamic_cast() in this hot path;
    and mixing QDev API within MR one. At least, can we cache this value
    once in memory_region_do_init() since we have access to @owner?

> +    if (mr->owner &&
> +        !mr->ram_device && !mr->ram && !mr->rom_device && !mr->readonly) {
> +        dev = (DeviceState *) object_dynamic_cast(mr->owner, TYPE_DEVICE);
> +        if (dev) {
> +            if (dev->mem_reentrancy_guard.engaged_in_io) {
> +                trace_memory_region_reentrant_io(get_cpu_index(), mr, addr, size);
> +                return MEMTX_ERROR;

MEMTX_ERROR is device-specific, I'm not sure it is right to return it
from this generic path. Maybe you meant MEMTX_ACCESS_ERROR?

> +            }
> +            dev->mem_reentrancy_guard.engaged_in_io = true;
> +        }
> +    }
> +
>       /* FIXME: support unaligned access? */
>       access_size = MAX(MIN(size, access_size_max), access_size_min);
>       access_mask = MAKE_64BIT_MASK(0, access_size * 8);
> @@ -556,6 +570,9 @@ static MemTxResult access_with_adjusted_size(hwaddr addr,
>                           access_mask, attrs);
>           }
>       }
> +    if (dev) {
> +        dev->mem_reentrancy_guard.engaged_in_io = false;
> +    }
>       return r;
>   }