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AHgh+RrC/WeiBvzbgWZzjmkaym3NCk+KumNsaUsowiVbrfzrfAVVGjNhhhuws97mT2WoMFIgnRJkxhiztbKl@lists.linux.dev X-Gm-Message-State: AOJu0YxvG2/hM1chQ0m3BFxO0DnPs0OKfF8dgpoQGJAPLiflQEu922b8 G3AfWU6RpzdZrgdRpd50d55buoBp2fKjsD5os5EtRxdqRlm8i7SKJYJtenmc1x93eQaQ3YuqTYP 2S7/QNA== X-Received: from plnx15.prod.google.com ([2002:a17:902:820f:b0:2c9:9db6:88e]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:903:2ec6:b0:2c9:e846:a57e with SMTP id d9443c01a7336-2ca90c2d830mr24586315ad.0.1782940958029; Wed, 01 Jul 2026 14:22:38 -0700 (PDT) Date: Wed, 1 Jul 2026 14:22:37 -0700 In-Reply-To: Precedence: bulk X-Mailing-List: linux-coco@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260630213711.479692-1-seanjc@google.com> <20260630213711.479692-2-seanjc@google.com> Message-ID: Subject: Re: [PATCH v2 1/2] KVM: SEV: Explicitly disallow NULL user address for SNP_LAUNCH_UPDATE From: Sean Christopherson To: Ackerley Tng Cc: Paolo Bonzini , Kiryl Shutsemau , Dave Hansen , Rick Edgecombe , kvm@vger.kernel.org, x86@kernel.org, linux-coco@lists.linux.dev, linux-kernel@vger.kernel.org, Sashiko Bot , Joerg Roedel , Yan Zhao Content-Type: text/plain; charset="us-ascii" On Wed, Jul 01, 2026, Ackerley Tng wrote: > Sean Christopherson writes: > > diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c > > index 74fb15551e83..621a2eaa58f2 100644 > > --- a/arch/x86/kvm/svm/sev.c > > +++ b/arch/x86/kvm/svm/sev.c > > @@ -2330,9 +2330,6 @@ static int sev_gmem_post_populate(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn, > > int level; > > int ret; > > > > - if (WARN_ON_ONCE(sev_populate_args->type != KVM_SEV_SNP_PAGE_TYPE_ZERO && !src_page)) > > - return -EINVAL; > > - > > ret = snp_lookup_rmpentry((u64)pfn, &assigned, &level); > > if (ret || assigned) { > > pr_debug("%s: Failed to ensure GFN 0x%llx RMP entry is initial shared state, ret: %d assigned: %d\n", > > @@ -2421,10 +2418,12 @@ static int snp_launch_update(struct kvm *kvm, struct kvm_sev_cmd *argp) > > params.type != KVM_SEV_SNP_PAGE_TYPE_CPUID)) > > return -EINVAL; > > > > - src = params.type == KVM_SEV_SNP_PAGE_TYPE_ZERO ? NULL : u64_to_user_ptr(params.uaddr); > > - > > - if (!PAGE_ALIGNED(src)) > > + if (params.type == KVM_SEV_SNP_PAGE_TYPE_ZERO) > > + src = NULL; > > + else if (!params.uaddr || !PAGE_ALIGNED(params.uaddr)) > > return -EINVAL; > > + else > > + src = u64_to_user_ptr(params.uaddr); > > > > I think separating validation from src assignment logic is better, like > > if (!PAGE_ALIGNED(params.uaddr)) > return -EINVAL; > > if (!params.uaddr && params.type != KVM_SEV_SNP_PAGE_TYPE_ZERO) > return -EINVAL; > > src = params.type == KVM_SEV_SNP_PAGE_TYPE_ZERO ? NULL : > u64_to_user_ptr(params.uaddr); > > This version is also slightly stricter, since it enforces that > params.uaddr is aligned even if params.type is ZERO. That's technically an ABI change since KVM fully ignored params.uaddr before this, i.e. is a potentially breaking change. As noted in the changelog, KVM should have required the address to be '0', but unfortunately we can't do that without breaking userspace. I highly doubt requiring the address to be page-aligned would break userspace, but KVM also gains nothing from such a requirement, and from an ABI perspective, placing restrictions on a value that is completely ignored is rather bizarre. > (unless the intention is to really not care about uaddr and permit an > unaligned uaddr?) I don't think there was any intention (which is the whole problem).