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[2003:d8:2f3f:f800:c101:5c9f:3bc9:3d08]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-40fc6de90desm2659224f8f.47.2025.09.25.04.00.45 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 25 Sep 2025 04:00:48 -0700 (PDT) Message-ID: Date: Thu, 25 Sep 2025 13:00:45 +0200 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v7 05/12] KVM: guest_memfd: Add flag to remove from direct map To: "Roy, Patrick" Cc: "pbonzini@redhat.com" , "corbet@lwn.net" , "maz@kernel.org" , "oliver.upton@linux.dev" , "joey.gouly@arm.com" , "suzuki.poulose@arm.com" , "yuzenghui@huawei.com" , "catalin.marinas@arm.com" , "will@kernel.org" , "tglx@linutronix.de" , "mingo@redhat.com" , "bp@alien8.de" , "dave.hansen@linux.intel.com" , "x86@kernel.org" , "hpa@zytor.com" , "luto@kernel.org" , "peterz@infradead.org" , "willy@infradead.org" , "akpm@linux-foundation.org" , "lorenzo.stoakes@oracle.com" , "Liam.Howlett@oracle.com" , "vbabka@suse.cz" , "rppt@kernel.org" , "surenb@google.com" , "mhocko@suse.com" , "song@kernel.org" , "jolsa@kernel.org" , "ast@kernel.org" , "daniel@iogearbox.net" , "andrii@kernel.org" , "martin.lau@linux.dev" , "eddyz87@gmail.com" , "yonghong.song@linux.dev" , "john.fastabend@gmail.com" , "kpsingh@kernel.org" , "sdf@fomichev.me" , "haoluo@google.com" , "jgg@ziepe.ca" , "jhubbard@nvidia.com" , "peterx@redhat.com" , "jannh@google.com" , "pfalcato@suse.de" , "shuah@kernel.org" , "seanjc@google.com" , "kvm@vger.kernel.org" , "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , "kvmarm@lists.linux.dev" , "linux-fsdevel@vger.kernel.org" , "linux-mm@kvack.org" , "bpf@vger.kernel.org" , "linux-kselftest@vger.kernel.org" , "Cali, Marco" , "Kalyazin, Nikita" , "Thomson, Jack" , "derekmn@amazon.co.uk" , "tabba@google.com" , "ackerleytng@google.com" References: <20250924151101.2225820-4-patrick.roy@campus.lmu.de> <20250924152214.7292-1-roypat@amazon.co.uk> <20250924152214.7292-2-roypat@amazon.co.uk> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 24.09.25 17:22, Roy, Patrick wrote: > Add GUEST_MEMFD_FLAG_NO_DIRECT_MAP flag for KVM_CREATE_GUEST_MEMFD() > ioctl. When set, guest_memfd folios will be removed from the direct map > after preparation, with direct map entries only restored when the folios > are freed. > > To ensure these folios do not end up in places where the kernel cannot > deal with them, set AS_NO_DIRECT_MAP on the guest_memfd's struct > address_space if GUEST_MEMFD_FLAG_NO_DIRECT_MAP is requested. > > Add KVM_CAP_GUEST_MEMFD_NO_DIRECT_MAP to let userspace discover whether > guest_memfd supports GUEST_MEMFD_FLAG_NO_DIRECT_MAP. Support depends on > guest_memfd itself being supported, but also on whether linux supports > manipulatomg the direct map at page granularity at all (possible most of > the time, outliers being arm64 where its impossible if the direct map > has been setup using hugepages, as arm64 cannot break these apart due to > break-before-make semantics, and powerpc, which does not select > ARCH_HAS_SET_DIRECT_MAP, though also doesn't support guest_memfd > anyway). > > Note that this flag causes removal of direct map entries for all > guest_memfd folios independent of whether they are "shared" or "private" > (although current guest_memfd only supports either all folios in the > "shared" state, or all folios in the "private" state if > GUEST_MEMFD_FLAG_MMAP is not set). The usecase for removing direct map > entries of also the shared parts of guest_memfd are a special type of > non-CoCo VM where, host userspace is trusted to have access to all of > guest memory, but where Spectre-style transient execution attacks > through the host kernel's direct map should still be mitigated. In this > setup, KVM retains access to guest memory via userspace mappings of > guest_memfd, which are reflected back into KVM's memslots via > userspace_addr. This is needed for things like MMIO emulation on x86_64 > to work. > > Direct map entries are zapped right before guest or userspace mappings > of gmem folios are set up, e.g. in kvm_gmem_fault_user_mapping() or > kvm_gmem_get_pfn() [called from the KVM MMU code]. The only place where > a gmem folio can be allocated without being mapped anywhere is > kvm_gmem_populate(), where handling potential failures of direct map > removal is not possible (by the time direct map removal is attempted, > the folio is already marked as prepared, meaning attempting to re-try > kvm_gmem_populate() would just result in -EEXIST without fixing up the > direct map state). These folios are then removed form the direct map > upon kvm_gmem_get_pfn(), e.g. when they are mapped into the guest later. > > Signed-off-by: Patrick Roy > --- > Documentation/virt/kvm/api.rst | 5 +++ > arch/arm64/include/asm/kvm_host.h | 12 ++++++ > include/linux/kvm_host.h | 6 +++ > include/uapi/linux/kvm.h | 2 + > virt/kvm/guest_memfd.c | 61 ++++++++++++++++++++++++++++++- > virt/kvm/kvm_main.c | 5 +++ > 6 files changed, 90 insertions(+), 1 deletion(-) > > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst > index c17a87a0a5ac..b52c14d58798 100644 > --- a/Documentation/virt/kvm/api.rst > +++ b/Documentation/virt/kvm/api.rst > @@ -6418,6 +6418,11 @@ When the capability KVM_CAP_GUEST_MEMFD_MMAP is supported, the 'flags' field > supports GUEST_MEMFD_FLAG_MMAP. Setting this flag on guest_memfd creation > enables mmap() and faulting of guest_memfd memory to host userspace. > > +When the capability KVM_CAP_GMEM_NO_DIRECT_MAP is supported, the 'flags' field > +supports GUEST_MEMFG_FLAG_NO_DIRECT_MAP. Setting this flag makes the guest_memfd > +instance behave similarly to memfd_secret, and unmaps the memory backing it from > +the kernel's address space after allocation. > + Do we want to document what the implication of that is? Meaning, limitations etc. I recall that we would need the user mapping for gmem slots to be properly set up. Is that still the case in this patch set? > When the KVM MMU performs a PFN lookup to service a guest fault and the backing > guest_memfd has the GUEST_MEMFD_FLAG_MMAP set, then the fault will always be > consumed from guest_memfd, regardless of whether it is a shared or a private > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h > index 2f2394cce24e..0bfd8e5fd9de 100644 > --- a/arch/arm64/include/asm/kvm_host.h > +++ b/arch/arm64/include/asm/kvm_host.h > @@ -19,6 +19,7 @@ > #include > #include > #include > +#include > #include > #include > #include > @@ -1706,5 +1707,16 @@ void compute_fgu(struct kvm *kvm, enum fgt_group_id fgt); > void get_reg_fixed_bits(struct kvm *kvm, enum vcpu_sysreg reg, u64 *res0, u64 *res1); > void check_feature_map(void); > > +#ifdef CONFIG_KVM_GUEST_MEMFD > +static inline bool kvm_arch_gmem_supports_no_direct_map(void) > +{ > + /* > + * Without FWB, direct map access is needed in kvm_pgtable_stage2_map(), > + * as it calls dcache_clean_inval_poc(). > + */ > + return can_set_direct_map() && cpus_have_final_cap(ARM64_HAS_STAGE2_FWB); > +} > +#define kvm_arch_gmem_supports_no_direct_map kvm_arch_gmem_supports_no_direct_map > +#endif /* CONFIG_KVM_GUEST_MEMFD */ > I strongly assume that the aarch64 support should be moved to a separate patch -- if possible, see below. > #endif /* __ARM64_KVM_HOST_H__ */ > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h > index 1d0585616aa3..73a15cade54a 100644 > --- a/include/linux/kvm_host.h > +++ b/include/linux/kvm_host.h > @@ -731,6 +731,12 @@ static inline bool kvm_arch_has_private_mem(struct kvm *kvm) > bool kvm_arch_supports_gmem_mmap(struct kvm *kvm); > #endif > > +#ifdef CONFIG_KVM_GUEST_MEMFD > +#ifndef kvm_arch_gmem_supports_no_direct_map > +#define kvm_arch_gmem_supports_no_direct_map can_set_direct_map > +#endif Hm, wouldn't it be better to have an opt-in per arch, and really only unlock the ones we know work (tested etc), explicitly in separate patches. [...] > > #include "kvm_mm.h" > > @@ -42,6 +45,44 @@ static int __kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slo > return 0; > } > > +#define KVM_GMEM_FOLIO_NO_DIRECT_MAP BIT(0) > + > +static bool kvm_gmem_folio_no_direct_map(struct folio *folio) > +{ > + return ((u64) folio->private) & KVM_GMEM_FOLIO_NO_DIRECT_MAP; > +} > + > +static int kvm_gmem_folio_zap_direct_map(struct folio *folio) > +{ > + if (kvm_gmem_folio_no_direct_map(folio)) > + return 0; > + > + int r = set_direct_map_valid_noflush(folio_page(folio, 0), folio_nr_pages(folio), > + false); > + > + if (!r) { > + unsigned long addr = (unsigned long) folio_address(folio); empty line missing. > + folio->private = (void *) ((u64) folio->private & KVM_GMEM_FOLIO_NO_DIRECT_MAP); > + flush_tlb_kernel_range(addr, addr + folio_size(folio)); > + } > + > + return r; > +} > + > +static void kvm_gmem_folio_restore_direct_map(struct folio *folio) > +{ > + /* > + * Direct map restoration cannot fail, as the only error condition > + * for direct map manipulation is failure to allocate page tables > + * when splitting huge pages, but this split would have already > + * happened in set_direct_map_invalid_noflush() in kvm_gmem_folio_zap_direct_map(). > + * Thus set_direct_map_valid_noflush() here only updates prot bits. > + */ > + if (kvm_gmem_folio_no_direct_map(folio)) > + set_direct_map_valid_noflush(folio_page(folio, 0), folio_nr_pages(folio), > + true); > +} > + > static inline void kvm_gmem_mark_prepared(struct folio *folio) > { > folio_mark_uptodate(folio); > @@ -324,13 +365,14 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf) > struct inode *inode = file_inode(vmf->vma->vm_file); > struct folio *folio; > vm_fault_t ret = VM_FAULT_LOCKED; > + int err; > > if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode)) > return VM_FAULT_SIGBUS; > > folio = kvm_gmem_get_folio(inode, vmf->pgoff); > if (IS_ERR(folio)) { > - int err = PTR_ERR(folio); > + err = PTR_ERR(folio); > > if (err == -EAGAIN) > return VM_FAULT_RETRY; > @@ -348,6 +390,13 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf) > kvm_gmem_mark_prepared(folio); > } > > + err = kvm_gmem_folio_zap_direct_map(folio); > + I'd drop this empty line here. > + if (err) { > + ret = vmf_error(err); > + goto out_folio; > + } > + > vmf->page = folio_file_page(folio, vmf->pgoff); > > out_folio: > @@ -435,6 +484,8 @@ static void kvm_gmem_free_folio(struct folio *folio) > kvm_pfn_t pfn = page_to_pfn(page); > int order = folio_order(folio); > > + kvm_gmem_folio_restore_direct_map(folio); > + > kvm_arch_gmem_invalidate(pfn, pfn + (1ul << order)); > } > > @@ -499,6 +550,9 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags) > /* Unmovable mappings are supposed to be marked unevictable as well. */ > WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping)); > > + if (flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP) > + mapping_set_no_direct_map(inode->i_mapping); > + > kvm_get_kvm(kvm); > gmem->kvm = kvm; > xa_init(&gmem->bindings); > @@ -523,6 +577,9 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args) > if (kvm_arch_supports_gmem_mmap(kvm)) > valid_flags |= GUEST_MEMFD_FLAG_MMAP; > > + if (kvm_arch_gmem_supports_no_direct_map()) > + valid_flags |= GUEST_MEMFD_FLAG_NO_DIRECT_MAP; > + > if (flags & ~valid_flags) > return -EINVAL; > > @@ -687,6 +744,8 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot, > if (!is_prepared) > r = kvm_gmem_prepare_folio(kvm, slot, gfn, folio); > > + kvm_gmem_folio_zap_direct_map(folio); > + > folio_unlock(folio); > > if (!r) > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > index 18f29ef93543..b5e702d95230 100644 > --- a/virt/kvm/kvm_main.c > +++ b/virt/kvm/kvm_main.c > @@ -65,6 +65,7 @@ > #include > > #include > +#include Likely not required here. -- Cheers David / dhildenb