From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nicholas Piggin Date: Thu, 24 Jun 2021 11:42:44 +0000 Subject: Re: [PATCH 0/6] KVM: Remove uses of struct page from x86 and arm64 MMU Message-Id: <1624534759.nj0ylor2eh.astroid@bobo.none> List-Id: References: <20210624035749.4054934-1-stevensd@google.com> <1624530624.8jff1f4u11.astroid@bobo.none> In-Reply-To: <1624530624.8jff1f4u11.astroid@bobo.none> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Aleksandar Markovic , Huacai Chen , Marc Zyngier , Paul Mackerras , Paolo Bonzini , David Stevens , Zhenyu Wang , Zhi Wang Cc: Alexandru Elisei , dri-devel@lists.freedesktop.org, intel-gfx@lists.freedesktop.org, intel-gvt-dev@lists.freedesktop.org, James Morse , Jim Mattson , Joerg Roedel , 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 , David Stevens , Suzuki K Poulose , Vitaly Kuznetsov , Wanpeng Li , Will Deacon 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. >> >> 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. >> >> This addresses CVE-2021-22543 on x86 and arm64. > > 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 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; } +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_struct *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. */ - kvm_get_pfn(pfn); + if (!kvm_try_get_pfn(pfn)) + r = -EFAULT; out: pte_unmap_unlock(ptep, ptl); *p_pfn = pfn; - return 0; + + return r; } /* -- 2.23.0