Re: [PATCH] virt: acrn: obtain pa from VMA with PFNMAP flag

[Daniel Vetter]

On Mon, Feb 28, 2022 at 05:22:12AM +0300, Yonghua Huang wrote:
>  acrn_vm_ram_map can't pin the user pages with VM_PFNMAP flag
>  by calling get_user_pages_fast(), the PA(physical pages)
>  may be mapped by kernel driver and set PFNMAP flag.
> 
>  This patch fixes logic to setup EPT mapping for PFN mapped RAM region
>  by checking the memory attribute before adding EPT mapping for them.
> 
> Fixes: 88f537d5e8dd ("virt: acrn: Introduce EPT mapping management")
> Signed-off-by: Yonghua Huang <yonghua.huang@xxxxxxxxx>
> Signed-off-by: Fei Li <fei1.li@xxxxxxxxx>
> ---
>  drivers/virt/acrn/mm.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/drivers/virt/acrn/mm.c b/drivers/virt/acrn/mm.c
> index c4f2e15c8a2b..3b1b1e7a844b 100644
> --- a/drivers/virt/acrn/mm.c
> +++ b/drivers/virt/acrn/mm.c
> @@ -162,10 +162,34 @@ int acrn_vm_ram_map(struct acrn_vm *vm, struct acrn_vm_memmap *memmap)
>  	void *remap_vaddr;
>  	int ret, pinned;
>  	u64 user_vm_pa;
> +	unsigned long pfn;
> +	struct vm_area_struct *vma;
>  
>  	if (!vm || !memmap)
>  		return -EINVAL;
>  
> +	mmap_read_lock(current->mm);
> +	vma = vma_lookup(current->mm, memmap->vma_base);
> +	if (vma && ((vma->vm_flags & VM_PFNMAP) != 0)) {
> +		if ((memmap->vma_base + memmap->len) > vma->vm_end) {
> +			mmap_read_unlock(current->mm);
> +			return -EINVAL;
> +		}
> +
> +		ret = follow_pfn(vma, memmap->vma_base, &pfn);

This races, don't use follow_pfn() and most definitely don't add new
users. In some cases follow_pte, but the pte/pfn is still only valid for
as long as you hold the pte spinlock.

> +		mmap_read_unlock(current->mm);

Definitely after here there's zero guarantees about this pfn and it could
point at anything.

Please fix, I tried pretty hard to get rid of follow_pfn(), but some of
them are just too hard to fix (e.g. kvm needs a pretty hug rewrite to get
it all sorted).

Cheers, Daniel

> +		if (ret < 0) {
> +			dev_dbg(acrn_dev.this_device,
> +				"Failed to lookup PFN at VMA:%pK.\n", (void *)memmap->vma_base);
> +			return ret;
> +		}
> +
> +		return acrn_mm_region_add(vm, memmap->user_vm_pa,
> +			 PFN_PHYS(pfn), memmap->len,
> +			 ACRN_MEM_TYPE_WB, memmap->attr);
> +	}
> +	mmap_read_unlock(current->mm);
> +
>  	/* Get the page number of the map region */
>  	nr_pages = memmap->len >> PAGE_SHIFT;
>  	pages = vzalloc(nr_pages * sizeof(struct page *));
> 
> base-commit: 73878e5eb1bd3c9656685ca60bc3a49d17311e0c
> -- 
> 2.25.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch