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[35.185.214.157]) by smtp.gmail.com with ESMTPSA id n9sm2973973pfu.152.2021.09.09.14.41.56 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 09 Sep 2021 14:41:56 -0700 (PDT) Date: Thu, 9 Sep 2021 21:41:52 +0000 From: Sean Christopherson To: Xiaoyao Li Cc: Paolo Bonzini , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , kvm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 6/7] KVM: VMX: Check Intel PT related CPUID leaves Message-ID: References: <20210827070249.924633-1-xiaoyao.li@intel.com> <20210827070249.924633-7-xiaoyao.li@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210827070249.924633-7-xiaoyao.li@intel.com> Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Fri, Aug 27, 2021, Xiaoyao Li wrote: > CPUID 0xD leaves reports the capabilities of Intel PT, e.g. it decides > which bits are valid to be set in MSR_IA32_RTIT_CTL, and reports the > number of PT ADDR ranges. > > KVM needs to check that guest CPUID values set by userspace doesn't > enable any bit which is not supported by bare metal. Otherwise, > 1. it will trigger vm-entry failure if hardware unsupported bit is > exposed to guest and set by guest. > 2. it triggers #GP when context switch PT MSRs if exposing more > RTIT_ADDR* MSRs than hardware capacity. > > Signed-off-by: Xiaoyao Li > --- > There is bit 31 of CPUID(0xD, 0).ECX that doesn't restrict any bit in > MSR_IA32_RTIT_CTL. If guest has different value than host, it won't > cause any vm-entry failure, but guest will parse the PT packet with > wrong format. > > I also check it to be same as host to ensure the virtualization correctness. > > Changes in v2: > - Call out that if configuring more PT ADDR MSRs than hardware, it can > cause #GP when context switch. > --- > arch/x86/kvm/cpuid.c | 25 +++++++++++++++++++++++++ > 1 file changed, 25 insertions(+) > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c > index 739be5da3bca..0c8e06a24156 100644 > --- a/arch/x86/kvm/cpuid.c > +++ b/arch/x86/kvm/cpuid.c > @@ -76,6 +76,7 @@ static inline struct kvm_cpuid_entry2 *cpuid_entry2_find( > static int kvm_check_cpuid(struct kvm_cpuid_entry2 *entries, int nent) > { > struct kvm_cpuid_entry2 *best; > + u32 eax, ebx, ecx, edx; > > /* > * The existing code assumes virtual address is 48-bit or 57-bit in the > @@ -89,6 +90,30 @@ static int kvm_check_cpuid(struct kvm_cpuid_entry2 *entries, int nent) > return -EINVAL; > } > > + /* > + * CPUID 0xD leaves tell Intel PT capabilities, which decides CPUID.0xD is XSAVE state, CPUID.0x14 is Intel PT. This series needs tests... > + * pt_desc.ctl_bitmask in later update_intel_pt_cfg(). > + * > + * pt_desc.ctl_bitmask decides the legal value for guest > + * MSR_IA32_RTIT_CTL. KVM cannot support PT capabilities beyond native, > + * otherwise it will trigger vm-entry failure if guest sets native > + * unsupported bits in MSR_IA32_RTIT_CTL. > + */ > + best = cpuid_entry2_find(entries, nent, 0xD, 0); > + if (best) { > + cpuid_count(0xD, 0, &eax, &ebx, &ecx, &edx); > + if (best->ebx & ~ebx || best->ecx & ~ecx) > + return -EINVAL; > + } > + best = cpuid_entry2_find(entries, nent, 0xD, 1); > + if (best) { > + cpuid_count(0xD, 0, &eax, &ebx, &ecx, &edx); > + if (((best->eax & 0x7) > (eax & 0x7)) || Ugh, looking at the rest of the code, even this isn't sufficient because pt_desc.guest.addr_{a,b} are hardcoded at 4 entries, i.e. running KVM on hardware with >4 entries will lead to buffer overflows. One option would be to bump that to the theoretical max of 15, which doesn't seem too horrible, especially if pt_desc as a whole is allocated on-demand, which it probably should be since it isn't exactly tiny (nor ubiquitous) A different option would be to let userspace define whatever it wants for guest CPUID, and instead cap nr_addr_ranges at min(host.cpuid, guest.cpuid, RTIT_ADDR_RANGE). Letting userspace generate a bad MSR_IA32_RTIT_CTL is not problematic, there are plenty of ways userspace can deliberately trigger VM-Entry failure due to invalid guest state (even if this is a VM-Fail condition, it's not a danger to KVM). > + ((best->eax & ~eax) >> 16) || > + (best->ebx & ~ebx)) > + return -EINVAL; > + } > + > return 0; > } > > -- > 2.27.0 >