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AJvYcCWWJNUrIZko6zgUPT9DMf4u42HmHimT4bUsDNYELxxelTRq8aTd+Pbo3pGSuXuu5ilhbmM=@vger.kernel.org X-Gm-Message-State: AOJu0YyvtC9v8qdm+tUiRWcfbRl7zEmkIE0fBoRXS6Mt7i6pyMxN5wRB FmvYUwAys1rmgn/3VXCMA+jwrFr01s2ADZ54d992nmL9t1+8Nelo31GGaaE1XHC7o0IpiIPhwyi 6Gw== X-Google-Smtp-Source: AGHT+IFEjTg3GcUU35aUT3dK0mgrxEOJsOyJ79AlQ4eiyt2SZQ54zCscuIHhjKiyu1sIAauYDAMKGjs8obg= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:9d:3983:ac13:c240]) (user=seanjc job=sendgmr) by 2002:a25:a1e9:0:b0:e30:b93a:b3e4 with SMTP id 3f1490d57ef6-e33025549e4mr19276.4.1730386486681; Thu, 31 Oct 2024 07:54:46 -0700 (PDT) Date: Thu, 31 Oct 2024 07:54:45 -0700 In-Reply-To: <3f158732a66829faaeb527a94b8df78d6173befa.camel@intel.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20240826022255.361406-1-binbin.wu@linux.intel.com> <20240826022255.361406-2-binbin.wu@linux.intel.com> <3f158732a66829faaeb527a94b8df78d6173befa.camel@intel.com> Message-ID: Subject: Re: [PATCH v3 1/2] KVM: x86: Check hypercall's exit to userspace generically From: Sean Christopherson To: Kai Huang Cc: "binbin.wu@linux.intel.com" , "yuan.yao@linux.intel.com" , "kvm@vger.kernel.org" , "pbonzini@redhat.com" , Xiaoyao Li , "linux-kernel@vger.kernel.org" , Rick Edgecombe , Isaku Yamahata Content-Type: text/plain; charset="us-ascii" On Thu, Oct 31, 2024, Kai Huang wrote: > > @@ -10111,12 +10111,21 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu) > > cpl = kvm_x86_call(get_cpl)(vcpu); > > > > ret = __kvm_emulate_hypercall(vcpu, nr, a0, a1, a2, a3, op_64_bit, cpl); > > - if (nr == KVM_HC_MAP_GPA_RANGE && !ret) > > - /* MAP_GPA tosses the request to the user space. */ > > + if (!ret) > > return 0; > > > > - if (!op_64_bit) > > + /* > > + * KVM's ABI with the guest is that '0' is success, and any other value > > + * is an error code. Internally, '0' == exit to userspace (see above) > > + * and '1' == success, as KVM's de facto standard return codes are that > > + * plus -errno == failure. Explicitly check for '1' as some PV error > > + * codes are positive values. > > + */ > > + if (ret == 1) > > + ret = 0; > > + else if (!op_64_bit) > > ret = (u32)ret; > > + > > kvm_rax_write(vcpu, ret); > > > > return kvm_skip_emulated_instruction(vcpu); > > > > It appears below two cases are not covered correctly? > > #ifdef CONFIG_X86_64 > case KVM_HC_CLOCK_PAIRING: > ret = kvm_pv_clock_pairing(vcpu, a0, a1); > break; > #endif > case KVM_HC_SEND_IPI: > if (!guest_pv_has(vcpu, KVM_FEATURE_PV_SEND_IPI)) > break; > > ret = kvm_pv_send_ipi(vcpu->kvm, a0, a1, a2, a3, op_64_bit); > break; > > Looking at the code, seems at least for KVM_HC_CLOCK_PAIRING, > kvm_pv_clock_pairing() returns 0 on success, and the upstream behaviour is not > routing to userspace but writing 0 to vcpu's RAX and returning to guest. With > the change above, it immediately returns to userspace w/o writing 0 to RAX. > > For KVM_HC_SEND_IPI, seems the upstream behaviour is the return value of > kvm_pv_send_ipi() is always written to vcpu's RAX reg, and we always just go > back to guest. With your change, the behaviour will be changed to: > > 1) when ret == 0, exit to userspace w/o writing 0 to vcpu's RAX, > 2) when ret == 1, it is converted to 0 and then written to RAX. > > This doesn't look like safe. Drat, I managed to space on the cases that didn't explicit set '0'. Hrm. My other idea was have an out-param to separate the return code intended for KVM from the return code intended for the guest. I generally dislike out-params, but trying to juggle a return value that multiplexes guest and host values seems like an even worse idea. Also completely untested... --- arch/x86/include/asm/kvm_host.h | 8 +++---- arch/x86/kvm/x86.c | 41 +++++++++++++++------------------ 2 files changed, 23 insertions(+), 26 deletions(-) diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index 6d9f763a7bb9..226df5c56811 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -2179,10 +2179,10 @@ static inline void kvm_clear_apicv_inhibit(struct kvm *kvm, kvm_set_or_clear_apicv_inhibit(kvm, reason, false); } -unsigned long __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr, - unsigned long a0, unsigned long a1, - unsigned long a2, unsigned long a3, - int op_64_bit, int cpl); +int __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr, + unsigned long a0, unsigned long a1, + unsigned long a2, unsigned long a3, + int op_64_bit, int cpl, unsigned long *ret); int kvm_emulate_hypercall(struct kvm_vcpu *vcpu); int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, u64 error_code, diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index e09daa3b157c..e9ae09f1b45b 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -9998,13 +9998,11 @@ static int complete_hypercall_exit(struct kvm_vcpu *vcpu) return kvm_skip_emulated_instruction(vcpu); } -unsigned long __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr, - unsigned long a0, unsigned long a1, - unsigned long a2, unsigned long a3, - int op_64_bit, int cpl) +int __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr, + unsigned long a0, unsigned long a1, + unsigned long a2, unsigned long a3, + int op_64_bit, int cpl, unsigned long *ret) { - unsigned long ret; - trace_kvm_hypercall(nr, a0, a1, a2, a3); if (!op_64_bit) { @@ -10016,15 +10014,15 @@ unsigned long __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr, } if (cpl) { - ret = -KVM_EPERM; + *ret = -KVM_EPERM; goto out; } - ret = -KVM_ENOSYS; + *ret = -KVM_ENOSYS; switch (nr) { case KVM_HC_VAPIC_POLL_IRQ: - ret = 0; + *ret = 0; break; case KVM_HC_KICK_CPU: if (!guest_pv_has(vcpu, KVM_FEATURE_PV_UNHALT)) @@ -10032,36 +10030,36 @@ unsigned long __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr, kvm_pv_kick_cpu_op(vcpu->kvm, a1); kvm_sched_yield(vcpu, a1); - ret = 0; + *ret = 0; break; #ifdef CONFIG_X86_64 case KVM_HC_CLOCK_PAIRING: - ret = kvm_pv_clock_pairing(vcpu, a0, a1); + *ret = kvm_pv_clock_pairing(vcpu, a0, a1); break; #endif case KVM_HC_SEND_IPI: if (!guest_pv_has(vcpu, KVM_FEATURE_PV_SEND_IPI)) break; - ret = kvm_pv_send_ipi(vcpu->kvm, a0, a1, a2, a3, op_64_bit); + *ret = kvm_pv_send_ipi(vcpu->kvm, a0, a1, a2, a3, op_64_bit); break; case KVM_HC_SCHED_YIELD: if (!guest_pv_has(vcpu, KVM_FEATURE_PV_SCHED_YIELD)) break; kvm_sched_yield(vcpu, a0); - ret = 0; + *ret = 0; break; case KVM_HC_MAP_GPA_RANGE: { u64 gpa = a0, npages = a1, attrs = a2; - ret = -KVM_ENOSYS; + *ret = -KVM_ENOSYS; if (!user_exit_on_hypercall(vcpu->kvm, KVM_HC_MAP_GPA_RANGE)) break; if (!PAGE_ALIGNED(gpa) || !npages || gpa_to_gfn(gpa) + npages <= gpa_to_gfn(gpa)) { - ret = -KVM_EINVAL; + *ret = -KVM_EINVAL; break; } @@ -10080,13 +10078,13 @@ unsigned long __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr, return 0; } default: - ret = -KVM_ENOSYS; + *ret = -KVM_ENOSYS; break; } out: ++vcpu->stat.hypercalls; - return ret; + return 1; } EXPORT_SYMBOL_GPL(__kvm_emulate_hypercall); @@ -10094,7 +10092,7 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu) { unsigned long nr, a0, a1, a2, a3, ret; int op_64_bit; - int cpl; + int cpl, r; if (kvm_xen_hypercall_enabled(vcpu->kvm)) return kvm_xen_hypercall(vcpu); @@ -10110,10 +10108,9 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu) op_64_bit = is_64_bit_hypercall(vcpu); cpl = kvm_x86_call(get_cpl)(vcpu); - ret = __kvm_emulate_hypercall(vcpu, nr, a0, a1, a2, a3, op_64_bit, cpl); - if (nr == KVM_HC_MAP_GPA_RANGE && !ret) - /* MAP_GPA tosses the request to the user space. */ - return 0; + r = __kvm_emulate_hypercall(vcpu, nr, a0, a1, a2, a3, op_64_bit, cpl, &ret); + if (r <= r) + return r; if (!op_64_bit) ret = (u32)ret; base-commit: 675248928970d33f7fc8ca9851a170c98f4f1c4f --