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(p200300cbc703e2008511ed0fac2c42f7.dip0.t-ipconnect.de. [2003:cb:c703:e200:8511:ed0f:ac2c:42f7]) by smtp.gmail.com with ESMTPSA id f29sm986699ejj.209.2022.01.13.07.56.53 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 13 Jan 2022 07:56:54 -0800 (PST) Message-ID: <0893e873-20c4-7e07-e7e4-3971dbb79118@redhat.com> Date: Thu, 13 Jan 2022 16:56:53 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.4.0 Subject: Re: [PATCH v3 kvm/queue 01/16] mm/shmem: Introduce F_SEAL_INACCESSIBLE To: Chao Peng Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, qemu-devel@nongnu.org, Paolo Bonzini , Jonathan Corbet , Sean Christopherson , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Thomas Gleixner , Ingo Molnar , Borislav Petkov , x86@kernel.org, "H . Peter Anvin" , Hugh Dickins , Jeff Layton , "J . Bruce Fields" , Andrew Morton , Yu Zhang , "Kirill A . Shutemov" , luto@kernel.org, john.ji@intel.com, susie.li@intel.com, jun.nakajima@intel.com, dave.hansen@intel.com, ak@linux.intel.com References: <20211223123011.41044-1-chao.p.peng@linux.intel.com> <20211223123011.41044-2-chao.p.peng@linux.intel.com> <7eb40902-45dd-9193-37f1-efaca381529b@redhat.com> <20220106130638.GB43371@chaop.bj.intel.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: <20220106130638.GB43371@chaop.bj.intel.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 1FCA940005 X-Stat-Signature: 73ohb1hoaqei49o8d9mpyadbocca3naa Authentication-Results: imf11.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=N3zuWt+q; dmarc=pass (policy=none) header.from=redhat.com; spf=none (imf11.hostedemail.com: domain of david@redhat.com has no SPF policy when checking 170.10.133.124) smtp.mailfrom=david@redhat.com X-Rspamd-Server: rspam02 X-HE-Tag: 1642089417-882892 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 06.01.22 14:06, Chao Peng wrote: > On Tue, Jan 04, 2022 at 03:22:07PM +0100, David Hildenbrand wrote: >> On 23.12.21 13:29, Chao Peng wrote: >>> From: "Kirill A. Shutemov" >>> >>> Introduce a new seal F_SEAL_INACCESSIBLE indicating the content of >>> the file is inaccessible from userspace in any possible ways like >>> read(),write() or mmap() etc. >>> >>> It provides semantics required for KVM guest private memory support >>> that a file descriptor with this seal set is going to be used as the >>> source of guest memory in confidential computing environments such >>> as Intel TDX/AMD SEV but may not be accessible from host userspace. >>> >>> At this time only shmem implements this seal. >>> >>> Signed-off-by: Kirill A. Shutemov >>> Signed-off-by: Chao Peng >>> --- >>> include/uapi/linux/fcntl.h | 1 + >>> mm/shmem.c | 37 +++++++++++++++++++++++++++++++++++-- >>> 2 files changed, 36 insertions(+), 2 deletions(-) >>> >>> diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h >>> index 2f86b2ad6d7e..e2bad051936f 100644 >>> --- a/include/uapi/linux/fcntl.h >>> +++ b/include/uapi/linux/fcntl.h >>> @@ -43,6 +43,7 @@ >>> #define F_SEAL_GROW 0x0004 /* prevent file from growing */ >>> #define F_SEAL_WRITE 0x0008 /* prevent writes */ >>> #define F_SEAL_FUTURE_WRITE 0x0010 /* prevent future writes while mapped */ >>> +#define F_SEAL_INACCESSIBLE 0x0020 /* prevent file from accessing */ >> >> I think this needs more clarification: the file content can still be >> accessed using in-kernel mechanisms such as MEMFD_OPS for KVM. It >> effectively disallows traditional access to a file (read/write/mmap) >> that will result in ordinary MMU access to file content. >> >> Not sure how to best clarify that: maybe, prevent ordinary MMU access >> (e.g., read/write/mmap) to file content? > > Or: prevent userspace access (e.g., read/write/mmap) to file content? The issue with that phrasing is that userspace will be able to access that content, just via a different mechanism eventually ... e.g., via the KVM MMU indirectly. If that makes it clearer what I mean :) >> >>> /* (1U << 31) is reserved for signed error codes */ >>> >>> /* >>> diff --git a/mm/shmem.c b/mm/shmem.c >>> index 18f93c2d68f1..faa7e9b1b9bc 100644 >>> --- a/mm/shmem.c >>> +++ b/mm/shmem.c >>> @@ -1098,6 +1098,10 @@ static int shmem_setattr(struct user_namespace *mnt_userns, >>> (newsize > oldsize && (info->seals & F_SEAL_GROW))) >>> return -EPERM; >>> >>> + if ((info->seals & F_SEAL_INACCESSIBLE) && >>> + (newsize & ~PAGE_MASK)) >>> + return -EINVAL; >>> + >> >> What happens when sealing and there are existing mmaps? > > I think this is similar to ftruncate, in either case we just allow that. > The existing mmaps will be unmapped and KVM will be notified to > invalidate the mapping in the secondary MMU as well. This assume we > trust the userspace even though it can not access the file content. Can't we simply check+forbid instead? -- Thanks, David / dhildenb