linux-kernel - Re: [PATCH] vfio iommu type1: Bypass the vma permission check in vfio_pin_pages

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Date:   Tue, 24 Nov 2020 13:12:28 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Jia He <justin.he@....com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vfio iommu type1: Bypass the vma permission check in
 vfio_pin_pages_remote()

Hi, Jia,

On Thu, Nov 19, 2020 at 10:27:37PM +0800, Jia He wrote:
> The permission of vfio iommu is different and incompatible with vma
> permission. If the iotlb->perm is IOMMU_NONE (e.g. qemu side), qemu will
> simply call unmap ioctl() instead of mapping. Hence vfio_dma_map() can't
> map a dma region with NONE permission.
> 
> This corner case will be exposed in coming virtio_fs cache_size
> commit [1]
>  - mmap(NULL, size, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
>    memory_region_init_ram_ptr()
>  - re-mmap the above area with read/write authority.

If iiuc here we'll remap the above PROT_NONE into PROT_READ|PROT_WRITE, then...

>  - vfio_dma_map() will be invoked when vfio device is hotplug added.

... here I'm slightly confused on why VFIO_IOMMU_MAP_DMA would encounter vma
check fail - aren't they already get rw permissions?

I'd appreciate if you could explain why vfio needs to dma map some PROT_NONE
pages after all, and whether QEMU would be able to postpone the vfio map of
those PROT_NONE pages until they got to become with RW permissions.

Thanks,

-- 
Peter Xu