RE: [for-4.15][PATCH v2 5/5] xen/iommu: x86: Clear the root page-table before freeing the page-tables

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Julien Grall <julien@xxxxxxx>
> Sent: Tuesday, February 9, 2021 11:28 PM
> 
> From: Julien Grall <jgrall@xxxxxxxxxx>
> 
> The new per-domain IOMMU page-table allocator will now free the
> page-tables when domain's resources are relinquished. However, the root
> page-table (i.e. hd->arch.pg_maddr) will not be cleared.

It's the pointer not the table itself.

> 
> Xen may access the IOMMU page-tables afterwards at least in the case of
> PV domain:
> 
> (XEN) Xen call trace:
> (XEN)    [<ffff82d04025b4b2>] R
> iommu.c#addr_to_dma_page_maddr+0x12e/0x1d8
> (XEN)    [<ffff82d04025b695>] F
> iommu.c#intel_iommu_unmap_page+0x5d/0xf8
> (XEN)    [<ffff82d0402695f3>] F iommu_unmap+0x9c/0x129
> (XEN)    [<ffff82d0402696a6>] F iommu_legacy_unmap+0x26/0x63
> (XEN)    [<ffff82d04033c5c7>] F mm.c#cleanup_page_mappings+0x139/0x144
> (XEN)    [<ffff82d04033c61d>] F put_page+0x4b/0xb3
> (XEN)    [<ffff82d04033c87f>] F put_page_from_l1e+0x136/0x13b
> (XEN)    [<ffff82d04033cada>] F devalidate_page+0x256/0x8dc
> (XEN)    [<ffff82d04033d396>] F mm.c#_put_page_type+0x236/0x47e
> (XEN)    [<ffff82d04033d64d>] F mm.c#put_pt_page+0x6f/0x80
> (XEN)    [<ffff82d04033d8d6>] F mm.c#put_page_from_l2e+0x8a/0xcf
> (XEN)    [<ffff82d04033cc27>] F devalidate_page+0x3a3/0x8dc
> (XEN)    [<ffff82d04033d396>] F mm.c#_put_page_type+0x236/0x47e
> (XEN)    [<ffff82d04033d64d>] F mm.c#put_pt_page+0x6f/0x80
> (XEN)    [<ffff82d04033d807>] F mm.c#put_page_from_l3e+0x8a/0xcf
> (XEN)    [<ffff82d04033cdf0>] F devalidate_page+0x56c/0x8dc
> (XEN)    [<ffff82d04033d396>] F mm.c#_put_page_type+0x236/0x47e
> (XEN)    [<ffff82d04033d64d>] F mm.c#put_pt_page+0x6f/0x80
> (XEN)    [<ffff82d04033d6c7>] F mm.c#put_page_from_l4e+0x69/0x6d
> (XEN)    [<ffff82d04033cf24>] F devalidate_page+0x6a0/0x8dc
> (XEN)    [<ffff82d04033d396>] F mm.c#_put_page_type+0x236/0x47e
> (XEN)    [<ffff82d04033d92e>] F put_page_type_preemptible+0x13/0x15
> (XEN)    [<ffff82d04032598a>] F domain.c#relinquish_memory+0x1ff/0x4e9
> (XEN)    [<ffff82d0403295f2>] F domain_relinquish_resources+0x2b6/0x36a
> (XEN)    [<ffff82d040205cdf>] F domain_kill+0xb8/0x141
> (XEN)    [<ffff82d040236cac>] F do_domctl+0xb6f/0x18e5
> (XEN)    [<ffff82d04031d098>] F pv_hypercall+0x2f0/0x55f
> (XEN)    [<ffff82d04039b432>] F lstar_enter+0x112/0x120
> 
> This will result to a use after-free and possibly an host crash or
> memory corruption.
> 
> Freeing the page-tables further down in domain_relinquish_resources()
> would not work because pages may not be released until later if another
> domain hold a reference on them.
> 
> Once all the PCI devices have been de-assigned, it is actually pointless
> to access modify the IOMMU page-tables. So we can simply clear the root
> page-table address.

Above two paragraphs are confusing to me. I don't know what exact change
is proposed until looking over the whole patch. Isn't it clearer to just say
"We should clear the root page table pointer in iommu_free_pgtables to
avoid use-after-free after all pgtables are moved to the free list"?

otherwise:

        Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>

> 
> Fixes: 3eef6d07d722 ("x86/iommu: convert VT-d code to use new page table
> allocator")
> Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
> 
> ---
>     Changes in v2:
>         - Introduce clear_root_pgtable()
>         - Move the patch later in the series
> ---
>  xen/drivers/passthrough/amd/pci_amd_iommu.c | 12 +++++++++++-
>  xen/drivers/passthrough/vtd/iommu.c         | 12 +++++++++++-
>  xen/drivers/passthrough/x86/iommu.c         |  6 ++++++
>  xen/include/xen/iommu.h                     |  1 +
>  4 files changed, 29 insertions(+), 2 deletions(-)
> 
> diff --git a/xen/drivers/passthrough/amd/pci_amd_iommu.c
> b/xen/drivers/passthrough/amd/pci_amd_iommu.c
> index 42b5a5a9bec4..81add0ba26b4 100644
> --- a/xen/drivers/passthrough/amd/pci_amd_iommu.c
> +++ b/xen/drivers/passthrough/amd/pci_amd_iommu.c
> @@ -381,9 +381,18 @@ static int amd_iommu_assign_device(struct domain
> *d, u8 devfn,
>      return reassign_device(pdev->domain, d, devfn, pdev);
>  }
> 
> +static void iommu_clear_root_pgtable(struct domain *d)
> +{
> +    struct domain_iommu *hd = dom_iommu(d);
> +
> +    spin_lock(&hd->arch.mapping_lock);
> +    hd->arch.amd.root_table = NULL;
> +    spin_unlock(&hd->arch.mapping_lock);
> +}
> +
>  static void amd_iommu_domain_destroy(struct domain *d)
>  {
> -    dom_iommu(d)->arch.amd.root_table = NULL;
> +    ASSERT(!dom_iommu(d)->arch.amd.root_table);
>  }
> 
>  static int amd_iommu_add_device(u8 devfn, struct pci_dev *pdev)
> @@ -565,6 +574,7 @@ static const struct iommu_ops __initconstrel
> _iommu_ops = {
>      .remove_device = amd_iommu_remove_device,
>      .assign_device  = amd_iommu_assign_device,
>      .teardown = amd_iommu_domain_destroy,
> +    .clear_root_pgtable = iommu_clear_root_pgtable,
>      .map_page = amd_iommu_map_page,
>      .unmap_page = amd_iommu_unmap_page,
>      .iotlb_flush = amd_iommu_flush_iotlb_pages,
> diff --git a/xen/drivers/passthrough/vtd/iommu.c
> b/xen/drivers/passthrough/vtd/iommu.c
> index d136fe36883b..e1871f6c2bc1 100644
> --- a/xen/drivers/passthrough/vtd/iommu.c
> +++ b/xen/drivers/passthrough/vtd/iommu.c
> @@ -1726,6 +1726,15 @@ out:
>      return ret;
>  }
> 
> +static void iommu_clear_root_pgtable(struct domain *d)
> +{
> +    struct domain_iommu *hd = dom_iommu(d);
> +
> +    spin_lock(&hd->arch.mapping_lock);
> +    hd->arch.vtd.pgd_maddr = 0;
> +    spin_unlock(&hd->arch.mapping_lock);
> +}
> +
>  static void iommu_domain_teardown(struct domain *d)
>  {
>      struct domain_iommu *hd = dom_iommu(d);
> @@ -1740,7 +1749,7 @@ static void iommu_domain_teardown(struct
> domain *d)
>          xfree(mrmrr);
>      }
> 
> -    hd->arch.vtd.pgd_maddr = 0;
> +    ASSERT(!hd->arch.vtd.pgd_maddr);
>  }
> 
>  static int __must_check intel_iommu_map_page(struct domain *d, dfn_t dfn,
> @@ -2719,6 +2728,7 @@ static struct iommu_ops __initdata vtd_ops = {
>      .remove_device = intel_iommu_remove_device,
>      .assign_device  = intel_iommu_assign_device,
>      .teardown = iommu_domain_teardown,
> +    .clear_root_pgtable = iommu_clear_root_pgtable,
>      .map_page = intel_iommu_map_page,
>      .unmap_page = intel_iommu_unmap_page,
>      .lookup_page = intel_iommu_lookup_page,
> diff --git a/xen/drivers/passthrough/x86/iommu.c
> b/xen/drivers/passthrough/x86/iommu.c
> index 82d770107a47..d3cdec6ee83f 100644
> --- a/xen/drivers/passthrough/x86/iommu.c
> +++ b/xen/drivers/passthrough/x86/iommu.c
> @@ -280,6 +280,12 @@ int iommu_free_pgtables(struct domain *d)
>      /* After this barrier no new page allocations can occur. */
>      spin_barrier(&hd->arch.pgtables.lock);
> 
> +    /*
> +     * Pages will be moved to the free list in a bit. So we want to
> +     * clear the root page-table to avoid any potential use after-free.
> +     */
> +    hd->platform_ops->clear_root_pgtable(d);
> +
>      while ( (pg = page_list_remove_head(&hd->arch.pgtables.list)) )
>      {
>          free_domheap_page(pg);
> diff --git a/xen/include/xen/iommu.h b/xen/include/xen/iommu.h
> index 863a68fe1622..d59ed7cbad43 100644
> --- a/xen/include/xen/iommu.h
> +++ b/xen/include/xen/iommu.h
> @@ -272,6 +272,7 @@ struct iommu_ops {
> 
>      int (*adjust_irq_affinities)(void);
>      void (*sync_cache)(const void *addr, unsigned int size);
> +    void (*clear_root_pgtable)(struct domain *d);
>  #endif /* CONFIG_X86 */
> 
>      int __must_check (*suspend)(void);
> --
> 2.17.1