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[35.247.111.240]) by smtp.gmail.com with ESMTPSA id d22sm2889002pjx.24.2021.04.06.09.07.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 06 Apr 2021 09:07:45 -0700 (PDT) Date: Tue, 6 Apr 2021 16:07:41 +0000 From: Sean Christopherson To: Emanuele Giuseppe Esposito Cc: kvm@vger.kernel.org, Paolo Bonzini , Jonathan Corbet , Vitaly Kuznetsov , Jim Mattson , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , Shuah Khan , Alexander Graf , Andrew Jones , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH v3 1/4] KVM: x86: Fix a spurious -E2BIG in KVM_GET_EMULATED_CPUID Message-ID: References: <20210406082642.20115-1-eesposit@redhat.com> <20210406082642.20115-2-eesposit@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210406082642.20115-2-eesposit@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On Tue, Apr 06, 2021, Emanuele Giuseppe Esposito wrote: > When retrieving emulated CPUID entries, check for an insufficient array > size if and only if KVM is actually inserting an entry. > If userspace has a priori knowledge of the exact array size, > KVM_GET_EMULATED_CPUID will incorrectly fail due to effectively requiring > an extra, unused entry. > > Signed-off-by: Emanuele Giuseppe Esposito Don't think it needs stable@, but I think it's worthwhile to add: Fixes: 433f4ba19041 ("KVM: x86: fix out-of-bounds write in KVM_GET_EMULATED_CPUID (CVE-2019-19332)") > --- > arch/x86/kvm/cpuid.c | 33 ++++++++++++++++----------------- > 1 file changed, 16 insertions(+), 17 deletions(-) > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c > index 6bd2f8b830e4..27059ddf9f0a 100644 > --- a/arch/x86/kvm/cpuid.c > +++ b/arch/x86/kvm/cpuid.c > @@ -567,34 +567,33 @@ static struct kvm_cpuid_entry2 *do_host_cpuid(struct kvm_cpuid_array *array, > > static int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func) > { > - struct kvm_cpuid_entry2 *entry; > - > - if (array->nent >= array->maxnent) > - return -E2BIG; > + struct kvm_cpuid_entry2 entry; > > - entry = &array->entries[array->nent]; > - entry->function = func; > - entry->index = 0; > - entry->flags = 0; > + memset(&entry, 0, sizeof(entry)); > + entry.function = func; Deep into nitpick territory... I think it makes sense to set entry.function only after the switch statement, that way it'll be a bit more obvious that the default case doesn't actually consume "entry". > > switch (func) { > case 0: > - entry->eax = 7; > - ++array->nent; > + entry.eax = 7; > break; > case 1: > - entry->ecx = F(MOVBE); > - ++array->nent; > + entry.ecx = F(MOVBE); > break; > case 7: > - entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX; > - entry->eax = 0; > - entry->ecx = F(RDPID); > - ++array->nent; > - default: > + entry.flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX; > + entry.eax = 0; > + entry.ecx = F(RDPID); > break; > + default: > + goto out; > } Maybe add a comment here to call out that the check is done if and only if there is a valid entry? > + if (array->nent >= array->maxnent) > + return -E2BIG; > + > + memcpy(&array->entries[array->nent++], &entry, sizeof(entry)); > + > +out: > return 0; > } > > -- > 2.30.2 >