From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7494D3A3E7B; Tue, 28 Apr 2026 23:25:19 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777418719; cv=none; b=l0wDXB3ZHDp0D/2wck6yZKRSM6Q8qcukmJelq4ny8Ooq0lbD8emGMs9d8Z6vtxWaVEy6/RezHvtVF+xk6ZHXVHG4R/ZXndk2UeNd9sTXewohdGo+hLsTmMBQ3tBtWvGlofZN3l+MI9l0N/0/XRyIZrq/3/8Gerklis1MmCViyAc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777418719; c=relaxed/simple; bh=m2bOW0DUVfOZna9REsWxASypSF6X/t2k8+SwYTvUrgc=; h=From:Date:Subject:MIME-Version:Content-Type:Message-Id:References: In-Reply-To:To:Cc; b=OUx82iUYw0i8aUPNFG38VB+mA88DQjIPXa9gQ56DRE8Bo5Apr61VP6uYjz+esW/MzWyHKMkAS9X8NaH3odxpxTDc+0gkcoQbSSX4XP2ORbxIg/hAmu3iWxD6OoR2uJLH3I9IRF259Edw+7uG9Em5W6Iv+ivelGjWsfze+64qfLo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=WJlnpg58; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="WJlnpg58" Received: by smtp.kernel.org (Postfix) with ESMTPS id 20591C4AF0F; Tue, 28 Apr 2026 23:25:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1777418719; bh=m2bOW0DUVfOZna9REsWxASypSF6X/t2k8+SwYTvUrgc=; h=From:Date:Subject:References:In-Reply-To:To:Cc:Reply-To:From; b=WJlnpg58o/wMHP8oA2epKzXvuu0F2uImhLPh4i+yCuIuUFUijKUcfQmk+2uN/pT7h 0cWGPK1gsPZsRGlY6uEWokR92R6JrFtTMZNz/hOPFIUyvlJDtPLmgpNGxkQ7BeooAj dXdSHhbv52GEyuZHyAoqqCj8Ggc6dPfwE6ETRPWQ7+uEs90xRB4/aZ5gLE5GdccoCd BepwaMv/5kjxDgHxS+6ycmPBriizSk/vDHl9jYlE2ON+EBB9gKhNOB3vl5A93+hgfg 6uNDvW5/2JlGSqtPjhVPmjyd6jIISjqN47JhWDQGSf+PsEuGcOs/6QTQ1ka9++XE3d ASdkLEjLfl5MA== Received: from aws-us-west-2-korg-lkml-1.web.codeaurora.org (localhost.localdomain [127.0.0.1]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0F5E0FF8875; Tue, 28 Apr 2026 23:25:19 +0000 (UTC) From: Ackerley Tng via B4 Relay Date: Tue, 28 Apr 2026 16:25:17 -0700 Subject: [PATCH RFC v5 22/53] KVM: guest_memfd: Apply content modes while setting memory attributes Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20260428-gmem-inplace-conversion-v5-22-d8608ccfca22@google.com> References: <20260428-gmem-inplace-conversion-v5-0-d8608ccfca22@google.com> In-Reply-To: <20260428-gmem-inplace-conversion-v5-0-d8608ccfca22@google.com> To: aik@amd.com, andrew.jones@linux.dev, binbin.wu@linux.intel.com, brauner@kernel.org, chao.p.peng@linux.intel.com, david@kernel.org, ira.weiny@intel.com, jmattson@google.com, jthoughton@google.com, michael.roth@amd.com, oupton@kernel.org, pankaj.gupta@amd.com, qperret@google.com, rick.p.edgecombe@intel.com, rientjes@google.com, shivankg@amd.com, steven.price@arm.com, tabba@google.com, willy@infradead.org, wyihan@google.com, yan.y.zhao@intel.com, forkloop@google.com, pratyush@kernel.org, suzuki.poulose@arm.com, aneesh.kumar@kernel.org, Paolo Bonzini , Sean Christopherson , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Jonathan Corbet , Shuah Khan , Shuah Khan , Vishal Annapurve , Andrew Morton , Chris Li , Kairui Song , Kemeng Shi , Nhat Pham , Baoquan He , Barry Song , Axel Rasmussen , Yuanchu Xie , Wei Xu , Youngjun Park , Qi Zheng , Shakeel Butt , Kiryl Shutsemau , Jason Gunthorpe , Vlastimil Babka Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mm@kvack.org, linux-coco@lists.linux.dev, Ackerley Tng X-Mailer: b4 0.14.3 X-Developer-Signature: v=1; a=ed25519-sha256; t=1777418714; l=8609; i=ackerleytng@google.com; s=20260225; h=from:subject:message-id; bh=MUDFg3cF6c9zZj9eh+GrJ3J5VFbXgeIVHlW3jinLVHw=; b=2XGqE3iV+Vi1bSn0hlNODYxtik3QyF9m9aUmkOk9DiJ6SyFSFBLsOf07/9slTvV8WlFojh1kw cm5zpBbGFbjByQhM6jLEcwU3cELZfDgCodbO76TBbTHf8UHrktag1mF X-Developer-Key: i=ackerleytng@google.com; a=ed25519; pk=sAZDYXdm6Iz8FHitpHeFlCMXwabodTm7p8/3/8xUxuU= X-Endpoint-Received: by B4 Relay for ackerleytng@google.com/20260225 with auth_id=649 X-Original-From: Ackerley Tng Reply-To: ackerleytng@google.com From: Ackerley Tng Provide defined memory content modes so that KVM can make guarantees about memory content after setting memory attributes, according to userspace requests. Suggested-by: Sean Christoperson Signed-off-by: Ackerley Tng --- Documentation/virt/kvm/api.rst | 61 ++++++++++++++++++++++++++++++++++++++++++ include/uapi/linux/kvm.h | 4 +++ virt/kvm/guest_memfd.c | 56 ++++++++++++++++++++++++++++++++++++-- 3 files changed, 119 insertions(+), 2 deletions(-) diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst index 6ce10c8ddb634..61b9974ba52e9 100644 --- a/Documentation/virt/kvm/api.rst +++ b/Documentation/virt/kvm/api.rst @@ -6573,6 +6573,8 @@ Errors: EAGAIN Some page within requested range had unexpected refcounts. The offset of the page will be returned in `error_offset`. ENOMEM Ran out of memory trying to track private/shared state + EOPNOTSUPP There is no way for KVM to guarantee in-memory contents as + requested. ========== =============================================================== KVM_SET_MEMORY_ATTRIBUTES2 is an extension to @@ -6621,6 +6623,65 @@ on the shared pages, such as refcounts taken by get_user_pages(), and try the ioctl again. A possible source of these long term refcounts is if the guest_memfd memory was pinned in IOMMU page tables. +By default, KVM makes no guarantees about the in-memory values after +memory is convert to/from shared/private. Optionally, userspace may +instruct KVM to ensure the contents of memory are zeroed or preserved, +e.g. to enable in-place sharing of data, or as an optimization to +avoid having to re-zero memory when userspace could have relied on the +trusted entity to guarantee the memory will be zeroed as part of the +entire conversion process. + +The content modes available are as follows: + +``KVM_SET_MEMORY_ATTRIBUTES2_ZERO`` + + On conversion, KVM guarantees all entities that have "allowed" + access to the memory will read zeros. E.g. on private to shared + conversion, both trusted and untrusted code will read zeros. + + Zeroing is currently only supported for private-to-shared + conversions, as KVM in general is untrusted and thus cannot + guarantee the guest (or any trusted entity) will read zeros after + conversion. Note, some CoCo implementations do zero memory contents + such that the guest reads zeros after conversion, and the guest may + choose to rely on that behavior. However, that's a contract between + the trusted CoCo entity and the guest, not between KVM and the + guest. + +``KVM_SET_MEMORY_ATTRIBUTES2_PRESERVE`` + + On conversion, KVM guarantees memory contents will be preserved with + respect to the last written unencrypted value. As a concrete + example, if the host writes ``0xbeef`` to shared memory and converts + the memory to private, the guest will also read ``0xbeef``, even if + the in-memory data is encrypted as part of the conversion. And vice + versa, if the guest writes ``0xbeef`` to private memory and then + converts the memory to shared, the host (and guest) will read + ``0xbeef`` (if the memory is accessible). + +Note: These content modes apply to the entire requested range, not +just the parts of the range that underwent conversion. For example, if +this was the initial state: + + * [0x0000, 0x1000): shared + * [0x1000, 0x2000): private + * [0x2000, 0x3000): shared + +and range [0x0000, 0x3000) was set to shared, the content mode would +apply to all memory in [0x0000, 0x3000), not just the range that +underwent conversion [0x1000, 0x2000). + +Note: These content modes apply only to allocated memory. No +guarantees are made on offset ranges that do not have memory allocated +(yet). For example, if this was the initial state: + + * [0x0000, 0x1000): shared + * [0x1000, 0x2000): not allocated + * [0x2000, 0x3000): shared + +and range [0x0000, 0x3000) was set to shared, the content mode would +apply to only to offset ranges [0x0000, 0x1000) and [0x2000, 0x3000). + See also: :ref: `KVM_SET_MEMORY_ATTRIBUTES`. .. _kvm_run: diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h index f437fd0f1350c..c7cc6c22c2023 100644 --- a/include/uapi/linux/kvm.h +++ b/include/uapi/linux/kvm.h @@ -1652,6 +1652,10 @@ struct kvm_memory_attributes { /* Available with KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES */ #define KVM_SET_MEMORY_ATTRIBUTES2 _IOWR(KVMIO, 0xd2, struct kvm_memory_attributes2) +#define KVM_SET_MEMORY_ATTRIBUTES2_MODE_UNSPECIFIED 0 +#define KVM_SET_MEMORY_ATTRIBUTES2_ZERO (1ULL << 0) +#define KVM_SET_MEMORY_ATTRIBUTES2_PRESERVE (1ULL << 1) + struct kvm_memory_attributes2 { union { __u64 address; diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c index b0e4bb554cdf3..5c1db67e6fd35 100644 --- a/virt/kvm/guest_memfd.c +++ b/virt/kvm/guest_memfd.c @@ -699,6 +699,19 @@ u64 __weak kvm_arch_gmem_supported_content_modes(struct kvm *kvm, bool to_privat return 0; } +static bool kvm_gmem_content_mode_is_supported(struct kvm *kvm, + u64 content_mode, + bool to_private) +{ + if (content_mode == KVM_SET_MEMORY_ATTRIBUTES2_MODE_UNSPECIFIED) + return true; + + if (content_mode == KVM_SET_MEMORY_ATTRIBUTES2_ZERO && to_private) + return false; + + return kvm_arch_gmem_supported_content_modes(kvm, to_private) & content_mode; +} + int kvm_gmem_apply_content_mode_zero(struct inode *inode, pgoff_t start, pgoff_t end) { @@ -759,8 +772,26 @@ int __weak kvm_arch_gmem_apply_content_mode_preserve(struct kvm *kvm, return -EOPNOTSUPP; } +static int kvm_gmem_apply_content_mode(struct kvm *kvm, uint64_t content_mode, + struct inode *inode, pgoff_t start, + pgoff_t end) +{ + switch (content_mode) { + case KVM_SET_MEMORY_ATTRIBUTES2_MODE_UNSPECIFIED: + return kvm_arch_gmem_apply_content_mode_unspecified(kvm, inode, start, end); + case KVM_SET_MEMORY_ATTRIBUTES2_ZERO: + return kvm_arch_gmem_apply_content_mode_zero(kvm, inode, start, end); + case KVM_SET_MEMORY_ATTRIBUTES2_PRESERVE: + return kvm_arch_gmem_apply_content_mode_preserve(kvm, inode, start, end); + default: + WARN_ONCE(1, "Unexpected policy requested."); + return -EOPNOTSUPP; + } +} + static int __kvm_gmem_set_attributes(struct inode *inode, pgoff_t start, size_t nr_pages, uint64_t attrs, + struct kvm *kvm, uint64_t content_mode, pgoff_t *err_index) { bool to_private = attrs & KVM_MEMORY_ATTRIBUTE_PRIVATE; @@ -775,7 +806,21 @@ static int __kvm_gmem_set_attributes(struct inode *inode, pgoff_t start, filemap_invalidate_lock(mapping); + if (!kvm_gmem_content_mode_is_supported(kvm, content_mode, + to_private)) { + r = -EOPNOTSUPP; + *err_index = start; + goto out; + } + if (kvm_gmem_range_has_attributes(mt, start, nr_pages, attrs)) { + /* + * Even if no update is required to attributes, the + * requested content mode is applied. + */ + WARN_ON(kvm_gmem_apply_content_mode(kvm, content_mode, + inode, start, end)); + r = 0; goto out; } @@ -808,6 +853,9 @@ static int __kvm_gmem_set_attributes(struct inode *inode, pgoff_t start, if (!to_private) kvm_gmem_invalidate(inode, start, end); + WARN_ON(kvm_gmem_apply_content_mode(kvm, content_mode, inode, + start, end)); + mas_store_prealloc(&mas, xa_mk_value(attrs)); kvm_gmem_invalidate_end(inode, start, end); @@ -829,7 +877,11 @@ static long kvm_gmem_set_attributes(struct file *file, void __user *argp) if (copy_from_user(&attrs, argp, sizeof(attrs))) return -EFAULT; - if (attrs.flags) + if (attrs.flags & ~(KVM_SET_MEMORY_ATTRIBUTES2_ZERO | + KVM_SET_MEMORY_ATTRIBUTES2_PRESERVE)) + return -EINVAL; + if ((attrs.flags & KVM_SET_MEMORY_ATTRIBUTES2_ZERO) && + (attrs.flags & KVM_SET_MEMORY_ATTRIBUTES2_PRESERVE)) return -EINVAL; for (i = 0; i < ARRAY_SIZE(attrs.reserved); i++) { if (attrs.reserved[i]) @@ -849,7 +901,7 @@ static long kvm_gmem_set_attributes(struct file *file, void __user *argp) nr_pages = attrs.size >> PAGE_SHIFT; index = attrs.offset >> PAGE_SHIFT; r = __kvm_gmem_set_attributes(inode, index, nr_pages, attrs.attributes, - &err_index); + f->kvm, attrs.flags, &err_index); if (r) { attrs.error_offset = ((uint64_t)err_index) << PAGE_SHIFT; -- 2.54.0.545.g6539524ca2-goog