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Date:   Thu, 24 Jun 2021 21:42:44 +1000
From:   Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH 0/6] KVM: Remove uses of struct page from x86 and arm64
 MMU
To:     Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
        Huacai Chen <chenhuacai@kernel.org>,
        Marc Zyngier <maz@kernel.org>,
        Paul Mackerras <paulus@ozlabs.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        David Stevens <stevensd@chromium.org>,
        Zhenyu Wang <zhenyuw@linux.intel.com>,
        Zhi Wang <zhi.a.wang@intel.com>
Cc:     Alexandru Elisei <alexandru.elisei@arm.com>,
        dri-devel@lists.freedesktop.org, intel-gfx@lists.freedesktop.org,
        intel-gvt-dev@lists.freedesktop.org,
        James Morse <james.morse@arm.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvmarm@lists.cs.columbia.edu,
        kvm-ppc@vger.kernel.org, kvm@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        Sean Christopherson <seanjc@google.com>,
        David Stevens <stevensd@google.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Will Deacon <will@kernel.org>
References: <20210624035749.4054934-1-stevensd@google.com>
        <1624530624.8jff1f4u11.astroid@bobo.none>
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Excerpts from Nicholas Piggin's message of June 24, 2021 8:34 pm:
> Excerpts from David Stevens's message of June 24, 2021 1:57 pm:
>> KVM supports mapping VM_IO and VM_PFNMAP memory into the guest by using
>> follow_pte in gfn_to_pfn. However, the resolved pfns may not have
>> assoicated struct pages, so they should not be passed to pfn_to_page.
>> This series removes such calls from the x86 and arm64 secondary MMU. To
>> do this, this series modifies gfn_to_pfn to return a struct page in
>> addition to a pfn, if the hva was resolved by gup. This allows the
>> caller to call put_page only when necessated by gup.
>>=20
>> This series provides a helper function that unwraps the new return type
>> of gfn_to_pfn to provide behavior identical to the old behavior. As I
>> have no hardware to test powerpc/mips changes, the function is used
>> there for minimally invasive changes. Additionally, as gfn_to_page and
>> gfn_to_pfn_cache are not integrated with mmu notifier, they cannot be
>> easily changed over to only use pfns.
>>=20
>> This addresses CVE-2021-22543 on x86 and arm64.
>=20
> Does this fix the problem? (untested I don't have a POC setup at hand,
> but at least in concept)

This one actually compiles at least. Unfortunately I don't have much=20
time in the near future to test, and I only just found out about this
CVE a few hours ago.

---


It's possible to create a region which maps valid but non-refcounted
pages (e.g., tail pages of non-compound higher order allocations). These
host pages can then be returned by gfn_to_page, gfn_to_pfn, etc., family
of APIs, which take a reference to the page, which takes it from 0 to 1.
When the reference is dropped, this will free the page incorrectly.

Fix this by only taking a reference on the page if it was non-zero,
which indicates it is participating in normal refcounting (and can be
released with put_page).

---
 virt/kvm/kvm_main.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 6a6bc7af0e28..46fb042837d2 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2055,6 +2055,13 @@ static bool vma_is_valid(struct vm_area_struct *vma,=
 bool write_fault)
 	return true;
 }
=20
+static int kvm_try_get_pfn(kvm_pfn_t pfn)
+{
+	if (kvm_is_reserved_pfn(pfn))
+		return 1;
+	return get_page_unless_zero(pfn_to_page(pfn));
+}
+
 static int hva_to_pfn_remapped(struct vm_area_struct *vma,
 			       unsigned long addr, bool *async,
 			       bool write_fault, bool *writable,
@@ -2104,13 +2111,21 @@ static int hva_to_pfn_remapped(struct vm_area_struc=
t *vma,
 	 * Whoever called remap_pfn_range is also going to call e.g.
 	 * unmap_mapping_range before the underlying pages are freed,
 	 * causing a call to our MMU notifier.
+	 *
+	 * Certain IO or PFNMAP mappings can be backed with valid
+	 * struct pages, but be allocated without refcounting e.g.,
+	 * tail pages of non-compound higher order allocations, which
+	 * would then underflow the refcount when the caller does the
+	 * required put_page. Don't allow those pages here.
 	 */=20
-	kvm_get_pfn(pfn);
+	if (!kvm_try_get_pfn(pfn))
+		r =3D -EFAULT;
=20
 out:
 	pte_unmap_unlock(ptep, ptl);
 	*p_pfn =3D pfn;
-	return 0;
+
+	return r;
 }
=20
 /*
--=20
2.23.0