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

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Date:   Thu, 19 Nov 2020 22:27:37 +0800
From:   Jia He <justin.he@....com>
To:     Alex Williamson <alex.williamson@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jia He <justin.he@....com>
Subject: [PATCH] vfio iommu type1: Bypass the vma permission check in vfio_pin_pages_remote()

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

qemu:
vfio_listener_region_add()
	vfio_dma_map(..., readonly=false)
		map.flags is set to VFIO_DMA_MAP_FLAG_READ|VFIO_..._WRITE
		ioctl(VFIO_IOMMU_MAP_DMA)

kernel:
vfio_dma_do_map()
	vfio_pin_map_dma()
		vfio_pin_pages_remote()
			vaddr_get_pfn()
			...
				check_vma_flags() failed! because
				vm_flags hasn't VM_WRITE && gup_flags
				has FOLL_WRITE

It will report error in qemu log when hotplug adding(vfio) a nvme disk
to qemu guest on an Ampere EMAG server:
"VFIO_MAP_DMA failed: Bad address"

[1] https://gitlab.com/virtio-fs/qemu/-/blob/virtio-fs-dev/hw/virtio/vhost-user-fs.c#L502

Signed-off-by: Jia He <justin.he@....com>
---
 drivers/vfio/vfio_iommu_type1.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index 67e827638995..33faa6b7dbd4 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -453,7 +453,8 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
 		flags |= FOLL_WRITE;
 
 	mmap_read_lock(mm);
-	ret = pin_user_pages_remote(mm, vaddr, 1, flags | FOLL_LONGTERM,
+	ret = pin_user_pages_remote(mm, vaddr, 1,
+				    flags | FOLL_LONGTERM | FOLL_FORCE,
 				    page, NULL, NULL);
 	if (ret == 1) {
 		*pfn = page_to_pfn(page[0]);
-- 
2.17.1