Re: [QEMU PATCH v5 07/13] softmmu/memory: enable automatic deallocation of memory regions

[Akihiko Odaki <akihiko.odaki@xxxxxxxxxx>]

On 2023/09/15 20:11, Huang Rui wrote:
> From: Xenia Ragiadakou <xenia.ragiadakou@xxxxxxx>
>
> When the memory region has a different life-cycle from that of her parent,
> could be automatically released, once has been unparent and once all of her
> references have gone away, via the object's free callback.
>
> However, currently, references to the memory region are held by its owner
> without first incrementing the memory region object's reference count.
> As a result, the automatic deallocation of the object, not taking into
> account those references, results in use-after-free memory corruption.
>
> This patch increases the reference count of an owned memory region object
> on each memory_region_ref() and decreases it on each memory_region_unref().
>
> Signed-off-by: Xenia Ragiadakou <xenia.ragiadakou@xxxxxxx>
> Signed-off-by: Huang Rui <ray.huang@xxxxxxx>
> ---
>
> V4 -> V5:
>      - ref/unref only owned memory regions (Akihiko)
>
>   softmmu/memory.c | 5 +++++
>   1 file changed, 5 insertions(+)
>
> diff --git a/softmmu/memory.c b/softmmu/memory.c
> index 7d9494ce70..15e1699750 100644
> --- a/softmmu/memory.c
> +++ b/softmmu/memory.c
> @@ -1800,6 +1800,9 @@ void memory_region_ref(MemoryRegion *mr)
>       /* MMIO callbacks most likely will access data that belongs
>        * to the owner, hence the need to ref/unref the owner whenever
>        * the memory region is in use.
> +     * Likewise, the owner keeps references to the memory region,
> +     * hence the need to ref/unref the memory region object to prevent
> +     * its automatic deallocation while still referenced by its owner.
This comment does not make sense. Traditionally no such automatic 
deallocation happens so the owner has been always required to free the 
memory region when it gets finalized.
"[QEMU PATCH v5 09/13] virtio-gpu: Handle resource blob commands" 
introduces a different kind of memory region, which can be freed anytime 
before the device gets finalized. Even in this case, the owner removes 
the reference to the memory owner by doing res->region = NULL;
Regards,
Akihiko Odaki