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Tue, 04 Aug 2026 05:05:51 -0700 (PDT) X-Received: by 2002:a05:600c:840f:b0:493:c42e:5be0 with SMTP id 5b1f17b1804b1-4980c60705amr298311535e9.0.1785845151462; Tue, 04 Aug 2026 05:05:51 -0700 (PDT) Received: from [192.168.10.48] ([151.95.34.92]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49949fd8df5sm88354535e9.9.2026.08.04.05.05.50 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 04 Aug 2026 05:05:50 -0700 (PDT) From: Paolo Bonzini To: linux-kernel@vger.kernel.org, kvm@vger.kernel.org Cc: Alex Williamson , bcm-kernel-feedback-list@broadcom.com, Boris Brezillon , Christian Koenig , David Hildenbrand , dri-devel@lists.freedesktop.org, Fei Li , Huang Rui , linux-mm@kvack.org, linux-s390@vger.kernel.org, Michal Hocko , Peter Xu , Sergio Lopez , Sean Christopherson , Thomas Zimmermann Subject: [PATCH v2 6/6] kvm: return -EFAULT for writes to !VM_WRITE IO mappings Date: Tue, 4 Aug 2026 14:05:28 +0200 Message-ID: <20260804120529.1730187-7-pbonzini@redhat.com> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260804120529.1730187-1-pbonzini@redhat.com> References: <20260804120529.1730187-1-pbonzini@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit KVM's behavior when the guest writes to a non-writable VMA is inconsistent. For regular, page-backed mappings it returns KVM_PFN_ERR_FAULT and thus returns -EFAULT to userspace (which is ABI, and relied upon by tests); for VM_IO/VM_PFNMAP mappings instead it returns KVM_PFN_ERR_RO_FAULT and thus exits to userspace with KVM_EXIT_MMIO. This behavior for VM_{IO,PFNMAP} was added by commit bd2fae8da794 ("KVM: do not assume PTE is writable after follow_pfn"), and even if it has been in place for five years it is unlikely that it is relied upon by userspace, since it is inconsistent with KVM itself. Change hva_to_pfn() to return KVM_PFN_ERR_FAULT for all non-writable VMAs, and restrict KVM_EXIT_MMIO to the case of an explicitly read-only memslot. Suggested-by: Sean Christopherson Signed-off-by: Paolo Bonzini --- virt/kvm/kvm_main.c | 11 +++-------- 1 file changed, 3 insertions(+), 8 deletions(-) diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index b7c21a48a45c..da5b0bb62590 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -2999,15 +2999,10 @@ kvm_pfn_t hva_to_pfn(struct kvm_follow_pfn *kfp) * or the page might be absent. */ - if (vma == NULL || unlikely(!(vma->vm_flags & VM_READ))) { + if (vma == NULL || + unlikely(!(vma->vm_flags & VM_READ)) || + ((kfp->flags & FOLL_WRITE) && unlikely(!(vma->vm_flags & VM_WRITE)))) { pfn = KVM_PFN_ERR_FAULT; - } else if ((kfp->flags & FOLL_WRITE) && unlikely(!(vma->vm_flags & VM_WRITE))) { - /* - * Exit to userspace for PROT_READ mappings in a writable - * memslot, as this is part of the API. - */ - pfn = vma->vm_flags & (VM_IO | VM_PFNMAP) ? KVM_PFN_ERR_RO_FAULT : - KVM_PFN_ERR_FAULT; } else if (vma->vm_flags & (VM_IO | VM_PFNMAP)) { r = hva_to_pfn_remapped(vma, kfp, &pfn); if (r == -EAGAIN) -- 2.55.0