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 2CC9629AB1A; Wed, 29 Jul 2026 00:35:09 +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=1785285309; cv=none; b=Re8ArM1V5JWr5d/MstONhR9iGGj9htW+W2wrIaG/pegf2ge8q6yZH0fu8gkDbVGnrlRccPTRMQy/IwEqPO4zaCsZNC3eRb1A8kCBgtItQguJPXU9LK1iiv2li6VSRBXojp6R3LFUd6s01sjYCsCOb++ipxGdp5Vda/N0VYE1xts= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785285309; c=relaxed/simple; bh=u3BZcC0GY/3lXX6mU2F+R3goOPNO5crd7wgdTocWeI4=; h=From:Date:Subject:MIME-Version:Content-Type:Message-Id:References: In-Reply-To:To:Cc; b=ty9nUaIw2emrpwO7wUWatIa3Y1NdprsmdaaTQSL8TPKctcqCKv9x2v0xWdLVgZEaN2ttAI4YJ49u9LFZBWMzVam3H1+S66slDUEAtaXN1RDJGdrhWig1GHuPSDtKiRYZkGNWwgJQCbp2Ohnkz2YmjMkq5VGt/i6eoeVSdS5xLLM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=p5ASZcfQ; 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="p5ASZcfQ" Received: by smtp.kernel.org (Postfix) with ESMTPS id B7C03C4DDFD; Wed, 29 Jul 2026 00:35:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1785285308; bh=u3BZcC0GY/3lXX6mU2F+R3goOPNO5crd7wgdTocWeI4=; h=From:Date:Subject:References:In-Reply-To:To:Cc:Reply-To:From; b=p5ASZcfQvuF51CdV7JHcL/oZEarovfqF7SGtLNxvHt8yPWstAPfTVdHV5dHiwS/uT FLxyl+V/1JdVwdZ5432rEBfPDLmSNpbgg4QmOVdc9q6yA4L7wVeIEBwaYNnhDts354 OFEu6XwvnL0ZOA4PlL+SJJ0xJChxnkszZJ4C2+NnlahB1eNhCejy2XsW3/X6ZtAUPk 3JhkSjVtNGoe5MQFwbIhbTGmd4bZVXa0A+NNJHeWjsPX8wHYX+HVGm5H5zE3Zjx1AB rvpdUhWBnbL9yZ3eASe9nlLie8DYkqlzo7bq93qEeL4YE0rinyydnQhY/+zE13QW8a WDfQUvoulJfUQ== 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 99BE7C54F52; Wed, 29 Jul 2026 00:35:08 +0000 (UTC) From: Ackerley Tng via B4 Relay Date: Tue, 28 Jul 2026 17:35:10 -0700 Subject: [PATCH v9 11/41] KVM: guest_memfd: Ensure pages are not in use before conversion Precedence: bulk X-Mailing-List: linux-doc@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: <20260728-gmem-inplace-conversion-v9-11-35f9aec2aed2@google.com> References: <20260728-gmem-inplace-conversion-v9-0-35f9aec2aed2@google.com> In-Reply-To: <20260728-gmem-inplace-conversion-v9-0-35f9aec2aed2@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, 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, liam@infradead.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 , Barry Song , Axel Rasmussen , Yuanchu Xie , Wei Xu , Youngjun Park , Qi Zheng , Shakeel Butt , Kiryl Shutsemau , Baoquan He , Jason Gunthorpe , John Hubbard , Peter Xu , Jason Gunthorpe , Vlastimil Babka , Baoquan He 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=1785285305; l=7235; i=ackerleytng@google.com; s=20260225; h=from:subject:message-id; bh=Bm3dCP0RZV+AH4I30jHVb8m6ilmyXuIUbLnn+589vrw=; b=7s4abQgDPUSNMcRlf7F6NBPTwDL9wlLCXDSSZXXrtsmlfk4FPzBiYQHrpTypT1HA7AZ/R6S2e zP09RyZML0JBzxs9MW0I4NrTzJVN9RPMhsCPGRihiuTHrS21WyDO1cU 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 When converting memory to private in guest_memfd, it is necessary to ensure that the pages are not currently being accessed by any other part of the kernel or userspace to avoid any current user writing to guest private memory. guest_memfd checks for unexpected refcounts to determine whether a page is still in use. The only expected refcounts after unmapping the range requested for conversion are those that are held by guest_memfd itself. Update the kvm_memory_attributes2 structure to include an error_offset field. This allows KVM to report the exact offset where a conversion failed to userspace. If the safety check fails, return -EAGAIN and copy the error_offset back to userspace so that it can potentially retry the operation or handle the failure gracefully. Update documentation to document the error_offset field and the possible -EAGAIN error. Suggested-by: David Hildenbrand Co-developed-by: Vishal Annapurve Signed-off-by: Vishal Annapurve Reviewed-by: Fuad Tabba Tested-by: Shivank Garg Signed-off-by: Ackerley Tng --- Documentation/virt/kvm/api.rst | 19 ++++++++++-- include/uapi/linux/kvm.h | 3 +- virt/kvm/guest_memfd.c | 67 ++++++++++++++++++++++++++++++++++++++---- 3 files changed, 81 insertions(+), 8 deletions(-) diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst index de8ab3db8796a..1de6fd5fc88c4 100644 --- a/Documentation/virt/kvm/api.rst +++ b/Documentation/virt/kvm/api.rst @@ -6574,7 +6574,7 @@ KVM_S390_KEYOP_SSKE :Capability: KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES :Architectures: all :Type: guest_memfd ioctl -:Parameters: struct kvm_memory_attributes2 (in) +:Parameters: struct kvm_memory_attributes2 (in/out) :Returns: 0 on success, <0 on error Errors: @@ -6583,6 +6583,8 @@ Errors: EINVAL The specified `offset` or `size` was invalid (e.g. not page aligned, causes an overflow, or size is zero). EFAULT The parameter address was invalid. + 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 ========== =============================================================== @@ -6596,6 +6598,7 @@ Attribute values are shared with KVM_SET_MEMORY_ATTRIBUTES. :: struct kvm_memory_attributes2 { + /* in */ union { __u64 address; __u64 offset; @@ -6603,7 +6606,9 @@ Attribute values are shared with KVM_SET_MEMORY_ATTRIBUTES. __u64 size; __u64 attributes; __u64 flags; - __u64 reserved[12]; + /* out */ + __u64 error_offset; + __u64 reserved[11]; }; #define KVM_MEMORY_ATTRIBUTE_PRIVATE (1ULL << 3) @@ -6625,6 +6630,16 @@ which includes operations such as unmapping pages from the host or stage-2 page tables, may result in side effects on memory contents that vary across different trusted firmware implementations. +If this ioctl returns -EAGAIN, the offset of the page with unexpected +refcounts will be returned in `error_offset`. This can occur if there +are transient refcounts on the pages, taken by other parts of the +kernel. + +Userspace is expected to figure out how to remove all known refcounts +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. + See also: :ref: `KVM_SET_MEMORY_ATTRIBUTES`. .. _kvm_run: diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h index 80985e28e3b21..129d6f6303251 100644 --- a/include/uapi/linux/kvm.h +++ b/include/uapi/linux/kvm.h @@ -1661,7 +1661,8 @@ struct kvm_memory_attributes2 { __u64 size; __u64 attributes; __u64 flags; - __u64 reserved[12]; + __u64 error_offset; + __u64 reserved[11]; }; #define KVM_MEMORY_ATTRIBUTE_PRIVATE (1ULL << 3) diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c index c9b5151f010e9..d3c813dcbf96d 100644 --- a/virt/kvm/guest_memfd.c +++ b/virt/kvm/guest_memfd.c @@ -529,8 +529,43 @@ static int kvm_gmem_mas_preallocate(struct ma_state *mas, u64 attributes, return mas_preallocate(mas, xa_mk_value(attributes), GFP_KERNEL); } +static bool kvm_gmem_is_safe_for_conversion(struct inode *inode, pgoff_t start, + size_t nr_pages, pgoff_t *err_index) +{ + struct address_space *mapping = inode->i_mapping; + const int filemap_get_folios_refcount = 1; + pgoff_t last = start + nr_pages - 1; + struct folio_batch fbatch; + bool safe = true; + pgoff_t next; + int i; + + folio_batch_init(&fbatch); + + next = start; + while (safe && filemap_get_folios(mapping, &next, last, &fbatch)) { + + for (i = 0; i < folio_batch_count(&fbatch); ++i) { + struct folio *folio = fbatch.folios[i]; + + if (folio_ref_count(folio) != + folio_nr_pages(folio) + filemap_get_folios_refcount) { + safe = false; + *err_index = max(start, folio->index); + break; + } + } + + folio_batch_release(&fbatch); + cond_resched(); + } + + return safe; +} + static int __kvm_gmem_set_attributes(struct inode *inode, pgoff_t start, - size_t nr_pages, uint64_t attrs) + size_t nr_pages, uint64_t attrs, + pgoff_t *err_index) { bool to_private = attrs & KVM_MEMORY_ATTRIBUTE_PRIVATE; struct address_space *mapping = inode->i_mapping; @@ -547,8 +582,21 @@ static int __kvm_gmem_set_attributes(struct inode *inode, pgoff_t start, mas_init(&mas, mt, start); r = kvm_gmem_mas_preallocate(&mas, attrs, start, nr_pages); - if (r) + if (r) { + *err_index = start; goto out; + } + + if (to_private) { + unmap_mapping_pages(mapping, start, nr_pages, false); + + if (!kvm_gmem_is_safe_for_conversion(inode, start, nr_pages, + err_index)) { + mas_destroy(&mas); + r = -EAGAIN; + goto out; + } + } /* * From this point on guest_memfd has performed necessary @@ -569,9 +617,10 @@ static long kvm_gmem_set_attributes(struct file *file, void __user *argp) struct gmem_file *f = file->private_data; struct inode *inode = file_inode(file); struct kvm_memory_attributes2 attrs; + pgoff_t err_index; size_t nr_pages; pgoff_t index; - int i; + int i, r; if (copy_from_user(&attrs, argp, sizeof(attrs))) return -EFAULT; @@ -597,8 +646,16 @@ static long kvm_gmem_set_attributes(struct file *file, void __user *argp) nr_pages = attrs.size >> PAGE_SHIFT; index = attrs.offset >> PAGE_SHIFT; - return __kvm_gmem_set_attributes(inode, index, nr_pages, - attrs.attributes); + r = __kvm_gmem_set_attributes(inode, index, nr_pages, attrs.attributes, + &err_index); + if (r) { + attrs.error_offset = ((uint64_t)err_index) << PAGE_SHIFT; + + if (copy_to_user(argp, &attrs, sizeof(attrs))) + return -EFAULT; + } + + return r; } static long kvm_gmem_ioctl(struct file *file, unsigned int ioctl, -- 2.55.0.508.g3f0d502094-goog