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AJvYcCVNYzaPGLxcs8bRMfGUrGHJCBVYfPwmcJRx9qpwJA1fC+m8CQshOPAnzKtih875XRxsohAYHyzAiQ==@kvack.org X-Gm-Message-State: AOJu0Yx0YQJex7nB5B0N/sOp+AMjakJMJli5Zqr8/Of2Ny9xj7+xRP3r LWNBTGqrHgDrNzHkOxSQ4Beew59p9W0RWpv6pvWszdz2jwUP6HmWnAVcMFfBAVa8JR23r5y+1u7 JR9hX2UKsBDbdVYv840GhhpXdrQ== X-Google-Smtp-Source: AGHT+IEZPq9JAZBfv/HDTaiqlxwio+gpwy5VWQkFScjzBWu4+U5mfVT+UUJR8YyZ7zO116a8IO9RMjZ+2gQY1usLMg== X-Received: from pfbcj22.prod.google.com ([2002:a05:6a00:2996:b0:747:a9de:9998]) (user=ackerleytng job=prod-delivery.src-stubby-dispatcher) by 2002:aa7:88c8:0:b0:736:3ea8:4805 with SMTP id d2e1a72fcca58-76034c56938mr377906b3a.7.1753208261268; Tue, 22 Jul 2025 11:17:41 -0700 (PDT) Date: Tue, 22 Jul 2025 11:17:39 -0700 In-Reply-To: Mime-Version: 1.0 References: <9502503f-e0c2-489e-99b0-94146f9b6f85@amd.com> <20250624130811.GB72557@ziepe.ca> <687a6483506f2_3c6f1d2945a@iweiny-mobl.notmuch> Message-ID: Subject: Re: [RFC PATCH v2 04/51] KVM: guest_memfd: Introduce KVM_GMEM_CONVERT_SHARED/PRIVATE ioctls From: Ackerley Tng To: Xu Yilun , Ira Weiny Cc: Yan Zhao , Vishal Annapurve , Jason Gunthorpe , Alexey Kardashevskiy , Fuad Tabba , kvm@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, x86@kernel.org, linux-fsdevel@vger.kernel.org, ajones@ventanamicro.com, akpm@linux-foundation.org, amoorthy@google.com, anthony.yznaga@oracle.com, anup@brainfault.org, aou@eecs.berkeley.edu, bfoster@redhat.com, binbin.wu@linux.intel.com, brauner@kernel.org, catalin.marinas@arm.com, chao.p.peng@intel.com, chenhuacai@kernel.org, dave.hansen@intel.com, david@redhat.com, dmatlack@google.com, dwmw@amazon.co.uk, erdemaktas@google.com, fan.du@intel.com, fvdl@google.com, graf@amazon.com, haibo1.xu@intel.com, hch@infradead.org, hughd@google.com, isaku.yamahata@intel.com, jack@suse.cz, james.morse@arm.com, jarkko@kernel.org, jgowans@amazon.com, jhubbard@nvidia.com, jroedel@suse.de, jthoughton@google.com, jun.miao@intel.com, kai.huang@intel.com, keirf@google.com, kent.overstreet@linux.dev, kirill.shutemov@intel.com, liam.merwick@oracle.com, maciej.wieczor-retman@intel.com, mail@maciej.szmigiero.name, maz@kernel.org, mic@digikod.net, michael.roth@amd.com, mpe@ellerman.id.au, muchun.song@linux.dev, nikunj@amd.com, nsaenz@amazon.es, oliver.upton@linux.dev, palmer@dabbelt.com, pankaj.gupta@amd.com, paul.walmsley@sifive.com, pbonzini@redhat.com, pdurrant@amazon.co.uk, peterx@redhat.com, pgonda@google.com, pvorel@suse.cz, qperret@google.com, quic_cvanscha@quicinc.com, quic_eberman@quicinc.com, quic_mnalajal@quicinc.com, quic_pderrin@quicinc.com, quic_pheragu@quicinc.com, quic_svaddagi@quicinc.com, quic_tsoni@quicinc.com, richard.weiyang@gmail.com, rick.p.edgecombe@intel.com, rientjes@google.com, roypat@amazon.co.uk, rppt@kernel.org, seanjc@google.com, shuah@kernel.org, steven.price@arm.com, steven.sistare@oracle.com, suzuki.poulose@arm.com, thomas.lendacky@amd.com, usama.arif@bytedance.com, vbabka@suse.cz, viro@zeniv.linux.org.uk, vkuznets@redhat.com, wei.w.wang@intel.com, will@kernel.org, willy@infradead.org, xiaoyao.li@intel.com, yilun.xu@intel.com, yuzenghui@huawei.com, zhiquan1.li@intel.com Content-Type: text/plain; charset="UTF-8" X-Rspam-User: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: CA8C880004 X-Stat-Signature: xmhs6y5wmpcqpmekd64dax4u4s6r7bqr X-HE-Tag: 1753208262-775221 X-HE-Meta: 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 vG9LeEgr 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 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: List-Subscribe: List-Unsubscribe: Xu Yilun writes: >> > > >> Yan, Yilun, would it work if, on conversion, >> > > >> >> > > >> 1. guest_memfd notifies IOMMU that a conversion is about to happen for a >> > > >> PFN range >> > > > >> > > > It is the Guest fw call to release the pinning. >> > > >> > > I see, thanks for explaining. >> > > >> > > > By the time VMM get the >> > > > conversion requirement, the page is already physically unpinned. So I >> > > > agree with Jason the pinning doesn't have to reach to iommu from SW POV. >> > > > >> > > >> > > If by the time KVM gets the conversion request, the page is unpinned, >> > > then we're all good, right? >> > >> > Yes, unless guest doesn't unpin the page first by mistake. >> >> Or maliciously? :-( > > Yes. > >> >> My initial response to this was that this is a bug and we don't need to be >> concerned with it. However, can't this be a DOS from one TD to crash the >> system if the host uses the private page for something else and the >> machine #MC's? > > I think we are already doing something to prevent vcpus from executing > then destroy VM, so no further TD accessing. But I assume there is > concern a TD could just leak a lot of resources, and we are > investigating if host can reclaim them. > > Thanks, > Yilun Sounds like a malicious guest could skip unpinning private memory, and guest_memfd's unmap will fail, leading to a KVM_BUG_ON() as Yan/Rick suggested here [1]. Actually it seems like a legacy guest would also lead to unmap failures and the KVM_BUG_ON(), since when TDX connect is enabled, the pinning mode is enforced, even for non-IO private pages? I hope your team's investigations find a good way for the host to reclaim memory, at least from dead TDs! Otherwise this would be an open hole for guests to leak a host's memory. Circling back to the original topic [2], it sounds like we're okay for IOMMU to *not* take any refcounts on pages and can rely on guest_memfd to keep the page around on behalf of the VM? [1] https://lore.kernel.org/all/diqzcya13x2j.fsf@ackerleytng-ctop.c.googlers.com/ [2] https://lore.kernel.org/all/CAGtprH_qh8sEY3s-JucW3n1Wvoq7jdVZDDokvG5HzPf0HV2=pg@mail.gmail.com/