From: Nadav Amit Date: Fri, 24 Oct 2014 17:07:17 +0200 Subject: [PATCH] KVM: x86: Handle errors when RIP is set during far jumps Far jmp/call/ret may fault while loading a new RIP. Currently KVM does not handle this case, and may result in failed vm-entry once the assignment is done. The tricky part of doing so is that loading the new CS affects the VMCS/VMCB state, so if we fail during loading the new RIP, we are left in unconsistent state. Therefore, this patch saves on 64-bit the old CS descriptor and restores it if loading RIP failed. This fixes CVE-2014-3647. Cc: stable@vger.kernel.org Signed-off-by: Nadav Amit Signed-off-by: Paolo Bonzini --- arch/x86/kvm/emulate.c | 118 ++++++++++++++++++++++++++++++++++++------------- 1 file changed, 88 insertions(+), 30 deletions(-) diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c index e52e74feedb8..1ac496349f39 100644 --- a/arch/x86/kvm/emulate.c +++ b/arch/x86/kvm/emulate.c @@ -1442,7 +1442,9 @@ static int write_segment_descriptor(struct x86_emulate_ctxt *ctxt, /* Does not support long mode */ static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt, - u16 selector, int seg, u8 cpl, bool in_task_switch) + u16 selector, int seg, u8 cpl, + bool in_task_switch, + struct desc_struct *desc) { struct desc_struct seg_desc, old_desc; u8 dpl, rpl; @@ -1574,6 +1576,8 @@ static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt, } load: ctxt->ops->set_segment(ctxt, selector, &seg_desc, base3, seg); + if (desc) + *desc = seg_desc; return X86EMUL_CONTINUE; exception: emulate_exception(ctxt, err_vec, err_code, true); @@ -1584,7 +1588,7 @@ static int load_segment_descriptor(struct x86_emulate_ctxt *ctxt, u16 selector, int seg) { u8 cpl = ctxt->ops->cpl(ctxt); - return __load_segment_descriptor(ctxt, selector, seg, cpl, false); + return __load_segment_descriptor(ctxt, selector, seg, cpl, false, NULL); } static void write_register_operand(struct operand *op) @@ -1978,17 +1982,31 @@ static int em_iret(struct x86_emulate_ctxt *ctxt) static int em_jmp_far(struct x86_emulate_ctxt *ctxt) { int rc; - unsigned short sel; + unsigned short sel, old_sel; + struct desc_struct old_desc, new_desc; + const struct x86_emulate_ops *ops = ctxt->ops; + u8 cpl = ctxt->ops->cpl(ctxt); + + /* Assignment of RIP may only fail in 64-bit mode */ + if (ctxt->mode == X86EMUL_MODE_PROT64) + ops->get_segment(ctxt, &old_sel, &old_desc, NULL, + VCPU_SREG_CS); memcpy(&sel, ctxt->src.valptr + ctxt->op_bytes, 2); - rc = load_segment_descriptor(ctxt, sel, VCPU_SREG_CS); + rc = __load_segment_descriptor(ctxt, sel, VCPU_SREG_CS, cpl, false, + &new_desc); if (rc != X86EMUL_CONTINUE) return rc; - ctxt->_eip = 0; - memcpy(&ctxt->_eip, ctxt->src.valptr, ctxt->op_bytes); - return X86EMUL_CONTINUE; + rc = assign_eip_far(ctxt, ctxt->src.val, new_desc.l); + if (rc != X86EMUL_CONTINUE) { + WARN_ON(!ctxt->mode != X86EMUL_MODE_PROT64); + /* assigning eip failed; restore the old cs */ + ops->set_segment(ctxt, old_sel, &old_desc, 0, VCPU_SREG_CS); + return rc; + } + return rc; } static int em_grp45(struct x86_emulate_ctxt *ctxt) @@ -2055,21 +2073,34 @@ static int em_ret(struct x86_emulate_ctxt *ctxt) static int em_ret_far(struct x86_emulate_ctxt *ctxt) { int rc; - unsigned long cs; + unsigned long eip, cs; + u16 old_cs; int cpl = ctxt->ops->cpl(ctxt); + struct desc_struct old_desc, new_desc; + const struct x86_emulate_ops *ops = ctxt->ops; + + if (ctxt->mode == X86EMUL_MODE_PROT64) + ops->get_segment(ctxt, &old_cs, &old_desc, NULL, + VCPU_SREG_CS); - rc = emulate_pop(ctxt, &ctxt->_eip, ctxt->op_bytes); + rc = emulate_pop(ctxt, &eip, ctxt->op_bytes); if (rc != X86EMUL_CONTINUE) return rc; - if (ctxt->op_bytes == 4) - ctxt->_eip = (u32)ctxt->_eip; rc = emulate_pop(ctxt, &cs, ctxt->op_bytes); if (rc != X86EMUL_CONTINUE) return rc; /* Outer-privilege level return is not implemented */ if (ctxt->mode >= X86EMUL_MODE_PROT16 && (cs & 3) > cpl) return X86EMUL_UNHANDLEABLE; - rc = load_segment_descriptor(ctxt, (u16)cs, VCPU_SREG_CS); + rc = __load_segment_descriptor(ctxt, (u16)cs, VCPU_SREG_CS, 0, false, + &new_desc); + if (rc != X86EMUL_CONTINUE) + return rc; + rc = assign_eip_far(ctxt, eip, new_desc.l); + if (rc != X86EMUL_CONTINUE) { + WARN_ON(!ctxt->mode != X86EMUL_MODE_PROT64); + ops->set_segment(ctxt, old_cs, &old_desc, 0, VCPU_SREG_CS); + } return rc; } @@ -2496,19 +2527,24 @@ static int load_state_from_tss16(struct x86_emulate_ctxt *ctxt, * Now load segment descriptors. If fault happens at this stage * it is handled in a context of new task */ - ret = __load_segment_descriptor(ctxt, tss->ldt, VCPU_SREG_LDTR, cpl, true); + ret = __load_segment_descriptor(ctxt, tss->ldt, VCPU_SREG_LDTR, cpl, + true, NULL); if (ret != X86EMUL_CONTINUE) return ret; - ret = __load_segment_descriptor(ctxt, tss->es, VCPU_SREG_ES, cpl, true); + ret = __load_segment_descriptor(ctxt, tss->es, VCPU_SREG_ES, cpl, + true, NULL); if (ret != X86EMUL_CONTINUE) return ret; - ret = __load_segment_descriptor(ctxt, tss->cs, VCPU_SREG_CS, cpl, true); + ret = __load_segment_descriptor(ctxt, tss->cs, VCPU_SREG_CS, cpl, + true, NULL); if (ret != X86EMUL_CONTINUE) return ret; - ret = __load_segment_descriptor(ctxt, tss->ss, VCPU_SREG_SS, cpl, true); + ret = __load_segment_descriptor(ctxt, tss->ss, VCPU_SREG_SS, cpl, + true, NULL); if (ret != X86EMUL_CONTINUE) return ret; - ret = __load_segment_descriptor(ctxt, tss->ds, VCPU_SREG_DS, cpl, true); + ret = __load_segment_descriptor(ctxt, tss->ds, VCPU_SREG_DS, cpl, + true, NULL); if (ret != X86EMUL_CONTINUE) return ret; @@ -2633,25 +2669,32 @@ static int load_state_from_tss32(struct x86_emulate_ctxt *ctxt, * Now load segment descriptors. If fault happenes at this stage * it is handled in a context of new task */ - ret = __load_segment_descriptor(ctxt, tss->ldt_selector, VCPU_SREG_LDTR, cpl, true); + ret = __load_segment_descriptor(ctxt, tss->ldt_selector, VCPU_SREG_LDTR, + cpl, true, NULL); if (ret != X86EMUL_CONTINUE) return ret; - ret = __load_segment_descriptor(ctxt, tss->es, VCPU_SREG_ES, cpl, true); + ret = __load_segment_descriptor(ctxt, tss->es, VCPU_SREG_ES, cpl, + true, NULL); if (ret != X86EMUL_CONTINUE) return ret; - ret = __load_segment_descriptor(ctxt, tss->cs, VCPU_SREG_CS, cpl, true); + ret = __load_segment_descriptor(ctxt, tss->cs, VCPU_SREG_CS, cpl, + true, NULL); if (ret != X86EMUL_CONTINUE) return ret; - ret = __load_segment_descriptor(ctxt, tss->ss, VCPU_SREG_SS, cpl, true); + ret = __load_segment_descriptor(ctxt, tss->ss, VCPU_SREG_SS, cpl, + true, NULL); if (ret != X86EMUL_CONTINUE) return ret; - ret = __load_segment_descriptor(ctxt, tss->ds, VCPU_SREG_DS, cpl, true); + ret = __load_segment_descriptor(ctxt, tss->ds, VCPU_SREG_DS, cpl, + true, NULL); if (ret != X86EMUL_CONTINUE) return ret; - ret = __load_segment_descriptor(ctxt, tss->fs, VCPU_SREG_FS, cpl, true); + ret = __load_segment_descriptor(ctxt, tss->fs, VCPU_SREG_FS, cpl, + true, NULL); if (ret != X86EMUL_CONTINUE) return ret; - ret = __load_segment_descriptor(ctxt, tss->gs, VCPU_SREG_GS, cpl, true); + ret = __load_segment_descriptor(ctxt, tss->gs, VCPU_SREG_GS, cpl, + true, NULL); if (ret != X86EMUL_CONTINUE) return ret; @@ -2934,24 +2977,39 @@ static int em_call_far(struct x86_emulate_ctxt *ctxt) u16 sel, old_cs; ulong old_eip; int rc; + struct desc_struct old_desc, new_desc; + const struct x86_emulate_ops *ops = ctxt->ops; + int cpl = ctxt->ops->cpl(ctxt); - old_cs = get_segment_selector(ctxt, VCPU_SREG_CS); old_eip = ctxt->_eip; + ops->get_segment(ctxt, &old_cs, &old_desc, NULL, VCPU_SREG_CS); memcpy(&sel, ctxt->src.valptr + ctxt->op_bytes, 2); - if (load_segment_descriptor(ctxt, sel, VCPU_SREG_CS)) + rc = __load_segment_descriptor(ctxt, sel, VCPU_SREG_CS, cpl, false, + &new_desc); + if (rc != X86EMUL_CONTINUE) return X86EMUL_CONTINUE; - ctxt->_eip = 0; - memcpy(&ctxt->_eip, ctxt->src.valptr, ctxt->op_bytes); + rc = assign_eip_far(ctxt, ctxt->src.val, new_desc.l); + if (rc != X86EMUL_CONTINUE) + goto fail; ctxt->src.val = old_cs; rc = em_push(ctxt); if (rc != X86EMUL_CONTINUE) - return rc; + goto fail; ctxt->src.val = old_eip; - return em_push(ctxt); + rc = em_push(ctxt); + /* If we failed, we tainted the memory, but the very least we should + restore cs */ + if (rc != X86EMUL_CONTINUE) + goto fail; + return rc; +fail: + ops->set_segment(ctxt, old_cs, &old_desc, 0, VCPU_SREG_CS); + return rc; + } static int em_ret_near_imm(struct x86_emulate_ctxt *ctxt) -- 1.9.3