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[213.175.37.10]) by smtp.gmail.com with ESMTPSA id r5sm25434161wrt.43.2020.02.03.04.55.40 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 03 Feb 2020 04:55:40 -0800 (PST) From: Vitaly Kuznetsov To: Sean Christopherson Cc: Wanpeng Li , Jim Mattson , Joerg Roedel , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Paolo Bonzini Subject: Re: [PATCH 01/61] KVM: x86: Return -E2BIG when KVM_GET_SUPPORTED_CPUID hits max entries In-Reply-To: <20200201185218.24473-2-sean.j.christopherson@intel.com> References: <20200201185218.24473-1-sean.j.christopherson@intel.com> <20200201185218.24473-2-sean.j.christopherson@intel.com> Date: Mon, 03 Feb 2020 13:55:40 +0100 Message-ID: <87mu9zomnn.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Sean Christopherson writes: > Fix a long-standing bug that causes KVM to return 0 instead of -E2BIG > when userspace's array is insufficiently sized. > > Note, while the Fixes: tag is accurate with respect to the immediate > bug, it's likely that similar bugs in KVM_GET_SUPPORTED_CPUID existed > prior to the refactoring, e.g. Qemu contains a workaround for the broken > KVM_GET_SUPPORTED_CPUID behavior that predates the buggy commit by over > two years. The Qemu workaround is also likely the main reason the bug > has gone unreported for so long. > > Qemu hack: > commit 76ae317f7c16aec6b469604b1764094870a75470 > Author: Mark McLoughlin > Date: Tue May 19 18:55:21 2009 +0100 > > kvm: work around supported cpuid ioctl() brokenness > > KVM_GET_SUPPORTED_CPUID has been known to fail to return -E2BIG > when it runs out of entries. Detect this by always trying again > with a bigger table if the ioctl() fills the table. > > Fixes: 831bf664e9c1f ("KVM: Refactor and simplify kvm_dev_ioctl_get_supported_cpuid") > Signed-off-by: Sean Christopherson > --- > arch/x86/kvm/cpuid.c | 7 ++++++- > 1 file changed, 6 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c > index b1c469446b07..47ce04762c20 100644 > --- a/arch/x86/kvm/cpuid.c > +++ b/arch/x86/kvm/cpuid.c > @@ -908,9 +908,14 @@ int kvm_dev_ioctl_get_cpuid(struct kvm_cpuid2 *cpuid, > goto out_free; > > limit = cpuid_entries[nent - 1].eax; > - for (func = ent->func + 1; func <= limit && nent < cpuid->nent && r == 0; ++func) > + for (func = ent->func + 1; func <= limit && r == 0; ++func) { > + if (nent >= cpuid->nent) { > + r = -E2BIG; > + goto out_free; > + } > r = do_cpuid_func(&cpuid_entries[nent], func, > &nent, cpuid->nent, type); > + } > > if (r) > goto out_free; Is fixing a bug a valid reason for breaking buggy userspace? :-) Personally, I think so. In particular, here the change is both the return value and the fact that we don't do copy_to_user() anymore so I think it's possible to meet a userspace which is going to get broken by the change. Reviewed-by: Vitaly Kuznetsov -- Vitaly