From a3641631d14571242eec0d30c9faa786cbf52d44 Mon Sep 17 00:00:00 2001 From: Wanpeng Li Date: Thu, 8 Jun 2017 01:22:07 -0700 Subject: [PATCH] KVM: cpuid: Fix read/write out-of-bounds vulnerability in cpuid emulation MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit If "i" is the last element in the vcpu->arch.cpuid_entries[] array, it potentially can be exploited the vulnerability. this will out-of-bounds read and write. Luckily, the effect is small: /* when no next entry is found, the current entry[i] is reselected */ for (j = i + 1; ; j = (j + 1) % nent) { struct kvm_cpuid_entry2 *ej = &vcpu->arch.cpuid_entries[j]; if (ej->function == e->function) { It reads ej->maxphyaddr, which is user controlled. However... ej->flags |= KVM_CPUID_FLAG_STATE_READ_NEXT; After cpuid_entries there is int maxphyaddr; struct x86_emulate_ctxt emulate_ctxt; /* 16-byte aligned */ So we have: - cpuid_entries at offset 1B50 (6992) - maxphyaddr at offset 27D0 (6992 + 3200 = 10192) - padding at 27D4...27DF - emulate_ctxt at 27E0 And it writes in the padding. Pfew, writing the ops field of emulate_ctxt would have been much worse. This patch fixes it by modding the index to avoid the out-of-bounds access. Worst case, i == j and ej->function == e->function, the loop can bail out. Reported-by: Moguofang Cc: Paolo Bonzini Cc: Radim Krčmář Cc: Guofang Mo Cc: stable@vger.kernel.org Signed-off-by: Wanpeng Li Signed-off-by: Paolo Bonzini --- arch/x86/kvm/cpuid.c | 20 +++++++++++--------- 1 file changed, 11 insertions(+), 9 deletions(-) diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c index a181ae76c71c..59ca2eea522c 100644 --- a/arch/x86/kvm/cpuid.c +++ b/arch/x86/kvm/cpuid.c @@ -780,18 +780,20 @@ out: static int move_to_next_stateful_cpuid_entry(struct kvm_vcpu *vcpu, int i) { struct kvm_cpuid_entry2 *e = &vcpu->arch.cpuid_entries[i]; - int j, nent = vcpu->arch.cpuid_nent; + struct kvm_cpuid_entry2 *ej; + int j = i; + int nent = vcpu->arch.cpuid_nent; e->flags &= ~KVM_CPUID_FLAG_STATE_READ_NEXT; /* when no next entry is found, the current entry[i] is reselected */ - for (j = i + 1; ; j = (j + 1) % nent) { - struct kvm_cpuid_entry2 *ej = &vcpu->arch.cpuid_entries[j]; - if (ej->function == e->function) { - ej->flags |= KVM_CPUID_FLAG_STATE_READ_NEXT; - return j; - } - } - return 0; /* silence gcc, even though control never reaches here */ + do { + j = (j + 1) % nent; + ej = &vcpu->arch.cpuid_entries[j]; + } while (ej->function != e->function); + + ej->flags |= KVM_CPUID_FLAG_STATE_READ_NEXT; + + return j; } /* find an entry with matching function, matching index (if needed), and that -- 2.20.1