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[60.242.147.73]) by smtp.gmail.com with ESMTPSA id h22sm2909006pfc.21.2021.06.24.05.57.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 24 Jun 2021 05:58:00 -0700 (PDT) Date: Thu, 24 Jun 2021 22:57:54 +1000 From: Nicholas Piggin Subject: Re: [PATCH 0/6] KVM: Remove uses of struct page from x86 and arm64 MMU To: Aleksandar Markovic , Huacai Chen , Marc Zyngier , Paul Mackerras , Paolo Bonzini , David Stevens , Zhenyu Wang , Zhi Wang References: <20210624035749.4054934-1-stevensd@google.com> <1624530624.8jff1f4u11.astroid@bobo.none> <1624534759.nj0ylor2eh.astroid@bobo.none> <0d3a699a-15eb-9f1b-0735-79d14736f38c@redhat.com> In-Reply-To: <0d3a699a-15eb-9f1b-0735-79d14736f38c@redhat.com> MIME-Version: 1.0 Message-Id: <1624539354.6zggpdrdbw.astroid@bobo.none> X-Mailman-Approved-At: Thu, 24 Jun 2021 10:03:53 -0400 Cc: Wanpeng Li , kvm@vger.kernel.org, David Stevens , intel-gfx@lists.freedesktop.org, linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, kvmarm@lists.cs.columbia.edu, Will Deacon , kvm-ppc@vger.kernel.org, Sean Christopherson , Vitaly Kuznetsov , linux-mips@vger.kernel.org, intel-gvt-dev@lists.freedesktop.org, Joerg Roedel , linux-arm-kernel@lists.infradead.org, Jim Mattson X-BeenThere: kvmarm@lists.cs.columbia.edu X-Mailman-Version: 2.1.14 Precedence: list List-Id: Where KVM/ARM decisions are made List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: kvmarm-bounces@lists.cs.columbia.edu Sender: kvmarm-bounces@lists.cs.columbia.edu Excerpts from Paolo Bonzini's message of June 24, 2021 10:41 pm: > On 24/06/21 13:42, Nicholas Piggin wrote: >> 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. > > And it also works (the reproducer gets an infinite stream of userspace > exits and especially does not crash). We can still go for David's > solution later since MMU notifiers are able to deal with this pages, but > it's a very nice patch for stable kernels. Oh nice, thanks for testing. How's this? Thanks, Nick --- KVM: Fix page ref underflow for regions with valid but non-refcounted pages 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). Signed-off-by: Nicholas Piggin --- 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 _______________________________________________ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm