From 6d1068b3a98519247d8ba4ec85cd40ac136dbdf9 Mon Sep 17 00:00:00 2001 From: Petr Matousek Date: Tue, 6 Nov 2012 19:24:07 +0100 Subject: [PATCH] KVM: x86: invalid opcode oops on SET_SREGS with OSXSAVE bit set (CVE-2012-4461) On hosts without the XSAVE support unprivileged local user can trigger oops similar to the one below by setting X86_CR4_OSXSAVE bit in guest cr4 register using KVM_SET_SREGS ioctl and later issuing KVM_RUN ioctl. invalid opcode: 0000 [#2] SMP Modules linked in: tun ip6table_filter ip6_tables ebtable_nat ebtables ... Pid: 24935, comm: zoog_kvm_monito Tainted: G D 3.2.0-3-686-pae EIP: 0060:[] EFLAGS: 00210246 CPU: 0 EIP is at kvm_arch_vcpu_ioctl_run+0x92a/0xd13 [kvm] EAX: 00000001 EBX: 000f387e ECX: 00000000 EDX: 00000000 ESI: 00000000 EDI: 00000000 EBP: ef5a0060 ESP: d7c63e70 DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068 Process zoog_kvm_monito (pid: 24935, ti=d7c62000 task=ed84a0c0 task.ti=d7c62000) Stack: 00000001 f70a1200 f8b940a9 ef5a0060 00000000 00200202 f8769009 00000000 ef5a0060 000f387e eda5c020 8722f9c8 00015bae 00000000 ed84a0c0 ed84a0c0 c12bf02d 0000ae80 ef7f8740 fffffffb f359b740 ef5a0060 f8b85dc1 0000ae80 Call Trace: [] ? kvm_arch_vcpu_ioctl_set_sregs+0x2fe/0x308 [kvm] ... [] ? syscall_call+0x7/0xb Code: 89 e8 e8 14 ee ff ff ba 00 00 04 00 89 e8 e8 98 48 ff ff 85 c0 74 1e 83 7d 48 00 75 18 8b 85 08 07 00 00 31 c9 8b 95 0c 07 00 00 <0f> 01 d1 c7 45 48 01 00 00 00 c7 45 1c 01 00 00 00 0f ae f0 89 EIP: [] kvm_arch_vcpu_ioctl_run+0x92a/0xd13 [kvm] SS:ESP 0068:d7c63e70 QEMU first retrieves the supported features via KVM_GET_SUPPORTED_CPUID and then sets them later. So guest's X86_FEATURE_XSAVE should be masked out on hosts without X86_FEATURE_XSAVE, making kvm_set_cr4 with X86_CR4_OSXSAVE fail. Userspaces that allow specifying guest cpuid with X86_FEATURE_XSAVE even on hosts that do not support it, might be susceptible to this attack from inside the guest as well. Allow setting X86_CR4_OSXSAVE bit only if host has XSAVE support. Signed-off-by: Petr Matousek Signed-off-by: Marcelo Tosatti --- arch/x86/kvm/cpuid.h | 3 +++ arch/x86/kvm/x86.c | 3 +++ 2 files changed, 6 insertions(+) diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h index a10e460..58fc514 100644 --- a/arch/x86/kvm/cpuid.h +++ b/arch/x86/kvm/cpuid.h @@ -24,6 +24,9 @@ static inline bool guest_cpuid_has_xsave(struct kvm_vcpu *vcpu) { struct kvm_cpuid_entry2 *best; + if (!static_cpu_has(X86_FEATURE_XSAVE)) + return 0; + best = kvm_find_cpuid_entry(vcpu, 1, 0); return best && (best->ecx & bit(X86_FEATURE_XSAVE)); } diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 224a7e7..4f76417 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -5781,6 +5781,9 @@ int kvm_arch_vcpu_ioctl_set_sregs(struct kvm_vcpu *vcpu, int pending_vec, max_bits, idx; struct desc_ptr dt; + if (!guest_cpuid_has_xsave(vcpu) && (sregs->cr4 & X86_CR4_OSXSAVE)) + return -EINVAL; + dt.size = sregs->idt.limit; dt.address = sregs->idt.base; kvm_x86_ops->set_idt(vcpu, &dt); -- 1.8.0