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[2003:cb:c733:f100:75e7:a0a4:9ac2:1abb]) by smtp.gmail.com with ESMTPSA id d16-20020a05600c34d000b00412eb0d455dsm266005wmq.41.2024.03.04.12.17.05 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 04 Mar 2024 12:17:07 -0800 (PST) Message-ID: Date: Mon, 4 Mar 2024 21:17:05 +0100 Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: folio_mmapped To: Sean Christopherson , Quentin Perret Cc: Matthew Wilcox , Fuad Tabba , kvm@vger.kernel.org, kvmarm@lists.linux.dev, pbonzini@redhat.com, chenhuacai@kernel.org, mpe@ellerman.id.au, anup@brainfault.org, paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, viro@zeniv.linux.org.uk, brauner@kernel.org, akpm@linux-foundation.org, xiaoyao.li@intel.com, yilun.xu@intel.com, chao.p.peng@linux.intel.com, jarkko@kernel.org, amoorthy@google.com, dmatlack@google.com, yu.c.zhang@linux.intel.com, isaku.yamahata@intel.com, mic@digikod.net, vbabka@suse.cz, vannapurve@google.com, ackerleytng@google.com, mail@maciej.szmigiero.name, michael.roth@amd.com, wei.w.wang@intel.com, liam.merwick@oracle.com, isaku.yamahata@gmail.com, kirill.shutemov@linux.intel.com, suzuki.poulose@arm.com, steven.price@arm.com, quic_mnalajal@quicinc.com, quic_tsoni@quicinc.com, quic_svaddagi@quicinc.com, quic_cvanscha@quicinc.com, quic_pderrin@quicinc.com, quic_pheragu@quicinc.com, catalin.marinas@arm.com, james.morse@arm.com, yuzenghui@huawei.com, oliver.upton@linux.dev, maz@kernel.org, will@kernel.org, keirf@google.com, linux-mm@kvack.org References: <40a8fb34-868f-4e19-9f98-7516948fc740@redhat.com> <20240226105258596-0800.eberman@hu-eberman-lv.qualcomm.com> <925f8f5d-c356-4c20-a6a5-dd7efde5ee86@redhat.com> <755911e5-8d4a-4e24-89c7-a087a26ec5f6@redhat.com> <99a94a42-2781-4d48-8b8c-004e95db6bb5@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 04.03.24 20:04, Sean Christopherson wrote: > On Mon, Mar 04, 2024, Quentin Perret wrote: >>> As discussed in the sub-thread, that might still be required. >>> >>> One could think about completely forbidding GUP on these mmap'ed >>> guest-memfds. But likely, there might be use cases in the future where you >>> want to use GUP on shared memory inside a guest_memfd. >>> >>> (the iouring example I gave might currently not work because >>> FOLL_PIN|FOLL_LONGTERM|FOLL_WRITE only works on shmem+hugetlb, and >>> guest_memfd will likely not be detected as shmem; 8ac268436e6d contains some >>> details) >> >> Perhaps it would be wise to start with GUP being forbidden if the >> current users do not need it (not sure if that is the case in Android, >> I'll check) ? We can always relax this constraint later when/if the >> use-cases arise, which is obviously much harder to do the other way >> around. > > +1000. At least on the KVM side, I would like to be as conservative as possible > when it comes to letting anything other than the guest access guest_memfd. So we'll have to do it similar to any occurrences of "secretmem" in gup.c. We'll have to see how to marry KVM guest_memfd with core-mm code similar to e.g., folio_is_secretmem(). IIRC, we might not be able to de-reference the actual mapping because it could get free concurrently ... That will then prohibit any kind of GUP access to these pages, including reading/writing for ptrace/debugging purposes, for core dumping purposes etc. But at least, you know that nobody was able to optain page references using GUP that might be used for reading/writing later. -- Cheers, David / dhildenb