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Fri, 26 Jul 2024 16:53:37 -0700 (PDT) Reply-To: Sean Christopherson Date: Fri, 26 Jul 2024 16:51:38 -0700 In-Reply-To: <20240726235234.228822-1-seanjc@google.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20240726235234.228822-1-seanjc@google.com> X-Mailer: git-send-email 2.46.0.rc1.232.g9752f9e123-goog Message-ID: <20240726235234.228822-30-seanjc@google.com> Subject: [PATCH v12 29/84] KVM: Pin (as in FOLL_PIN) pages during kvm_vcpu_map() From: Sean Christopherson To: Paolo Bonzini , Marc Zyngier , Oliver Upton , Tianrui Zhao , Bibo Mao , Huacai Chen , Michael Ellerman , Anup Patel , Paul Walmsley , Palmer Dabbelt , Albert Ou , Christian Borntraeger , Janosch Frank , Claudio Imbrenda , Sean Christopherson Cc: kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, David Matlack , David Stevens Pin, as in FOLL_PIN, pages when mapping them for direct access by KVM. As per Documentation/core-api/pin_user_pages.rst, writing to a page that was gotten via FOLL_GET is explicitly disallowed. Correct (uses FOLL_PIN calls): pin_user_pages() write to the data within the pages unpin_user_pages() INCORRECT (uses FOLL_GET calls): get_user_pages() write to the data within the pages put_page() Unfortunately, FOLL_PIN is a "private" flag, and so kvm_follow_pfn must use a one-off bool instead of being able to piggyback the "flags" field. Link: https://lwn.net/Articles/930667 Link: https://lore.kernel.org/all/cover.1683044162.git.lstoakes@gmail.com Signed-off-by: Sean Christopherson --- include/linux/kvm_host.h | 2 +- virt/kvm/kvm_main.c | 54 +++++++++++++++++++++++++++++----------- virt/kvm/kvm_mm.h | 7 ++++++ 3 files changed, 47 insertions(+), 16 deletions(-) diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index 8b5ac3305b05..3d4094ece479 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -280,7 +280,7 @@ struct kvm_host_map { * can be used as guest memory but they are not managed by host * kernel). */ - struct page *refcounted_page; + struct page *pinned_page; struct page *page; void *hva; kvm_pfn_t pfn; diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 255cbed83b40..4a9b99c11355 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -2824,9 +2824,12 @@ static kvm_pfn_t kvm_resolve_pfn(struct kvm_follow_pfn *kfp, struct page *page, */ if (pte) { pfn = pte_pfn(*pte); - page = kvm_pfn_to_refcounted_page(pfn); - if (page && !get_page_unless_zero(page)) - return KVM_PFN_ERR_FAULT; + + if (!kfp->pin) { + page = kvm_pfn_to_refcounted_page(pfn); + if (page && !get_page_unless_zero(page)) + return KVM_PFN_ERR_FAULT; + } } else { pfn = page_to_pfn(page); } @@ -2845,16 +2848,24 @@ static kvm_pfn_t kvm_resolve_pfn(struct kvm_follow_pfn *kfp, struct page *page, static bool hva_to_pfn_fast(struct kvm_follow_pfn *kfp, kvm_pfn_t *pfn) { struct page *page; + bool r; /* - * Fast pin a writable pfn only if it is a write fault request - * or the caller allows to map a writable pfn for a read fault - * request. + * Try the fast-only path when the caller wants to pin/get the page for + * writing. If the caller only wants to read the page, KVM must go + * down the full, slow path in order to avoid racing an operation that + * breaks Copy-on-Write (CoW), e.g. so that KVM doesn't end up pointing + * at the old, read-only page while mm/ points at a new, writable page. */ if (!((kfp->flags & FOLL_WRITE) || kfp->map_writable)) return false; - if (get_user_page_fast_only(kfp->hva, FOLL_WRITE, &page)) { + if (kfp->pin) + r = pin_user_pages_fast(kfp->hva, 1, FOLL_WRITE, &page) == 1; + else + r = get_user_page_fast_only(kfp->hva, FOLL_WRITE, &page); + + if (r) { *pfn = kvm_resolve_pfn(kfp, page, NULL, true); return true; } @@ -2883,10 +2894,21 @@ static int hva_to_pfn_slow(struct kvm_follow_pfn *kfp, kvm_pfn_t *pfn) struct page *page, *wpage; int npages; - npages = get_user_pages_unlocked(kfp->hva, 1, &page, flags); + if (kfp->pin) + npages = pin_user_pages_unlocked(kfp->hva, 1, &page, flags); + else + npages = get_user_pages_unlocked(kfp->hva, 1, &page, flags); if (npages != 1) return npages; + /* + * Pinning is mutually exclusive with opportunistically mapping a read + * fault as writable, as KVM should never pin pages when mapping memory + * into the guest (pinning is only for direct accesses from KVM). + */ + if (WARN_ON_ONCE(kfp->map_writable && kfp->pin)) + goto out; + /* map read fault as writable if possible */ if (!(flags & FOLL_WRITE) && kfp->map_writable && get_user_page_fast_only(kfp->hva, FOLL_WRITE, &wpage)) { @@ -2895,6 +2917,7 @@ static int hva_to_pfn_slow(struct kvm_follow_pfn *kfp, kvm_pfn_t *pfn) flags |= FOLL_WRITE; } +out: *pfn = kvm_resolve_pfn(kfp, page, NULL, flags & FOLL_WRITE); return npages; } @@ -3119,10 +3142,11 @@ int kvm_vcpu_map(struct kvm_vcpu *vcpu, gfn_t gfn, struct kvm_host_map *map) .slot = gfn_to_memslot(vcpu->kvm, gfn), .gfn = gfn, .flags = FOLL_WRITE, - .refcounted_page = &map->refcounted_page, + .refcounted_page = &map->pinned_page, + .pin = true, }; - map->refcounted_page = NULL; + map->pinned_page = NULL; map->page = NULL; map->hva = NULL; map->gfn = gfn; @@ -3159,16 +3183,16 @@ void kvm_vcpu_unmap(struct kvm_vcpu *vcpu, struct kvm_host_map *map, bool dirty) if (dirty) kvm_vcpu_mark_page_dirty(vcpu, map->gfn); - if (map->refcounted_page) { + if (map->pinned_page) { if (dirty) - kvm_release_page_dirty(map->refcounted_page); - else - kvm_release_page_clean(map->refcounted_page); + kvm_set_page_dirty(map->pinned_page); + kvm_set_page_accessed(map->pinned_page); + unpin_user_page(map->pinned_page); } map->hva = NULL; map->page = NULL; - map->refcounted_page = NULL; + map->pinned_page = NULL; } EXPORT_SYMBOL_GPL(kvm_vcpu_unmap); diff --git a/virt/kvm/kvm_mm.h b/virt/kvm/kvm_mm.h index d3ac1ba8ba66..acef3f5c582a 100644 --- a/virt/kvm/kvm_mm.h +++ b/virt/kvm/kvm_mm.h @@ -30,6 +30,13 @@ struct kvm_follow_pfn { /* FOLL_* flags modifying lookup behavior, e.g. FOLL_WRITE. */ unsigned int flags; + /* + * Pin the page (effectively FOLL_PIN, which is an mm/ internal flag). + * The page *must* be pinned if KVM will write to the page via a kernel + * mapping, e.g. via kmap(), mremap(), etc. + */ + bool pin; + /* * If non-NULL, try to get a writable mapping even for a read fault. * Set to true if a writable mapping was obtained.