From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753942AbdBOXLe (ORCPT ); Wed, 15 Feb 2017 18:11:34 -0500 Received: from shadbolt.e.decadent.org.uk ([88.96.1.126]:53697 "EHLO shadbolt.e.decadent.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751759AbdBOXLa (ORCPT ); Wed, 15 Feb 2017 18:11:30 -0500 Content-Type: text/plain; charset="UTF-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit MIME-Version: 1.0 From: Ben Hutchings To: linux-kernel@vger.kernel.org, stable@vger.kernel.org CC: akpm@linux-foundation.org, "Dmitry Vyukov" , "=?UTF-8?q?Radim=20Kr=C4=8Dm=C3=A1=C5=99?=" Date: Wed, 15 Feb 2017 22:41:40 +0000 Message-ID: X-Mailer: LinuxStableQueue (scripts by bwh) Subject: [PATCH 3.16 260/306] KVM: x86: drop error recovery in em_jmp_far and em_ret_far In-Reply-To: X-SA-Exim-Connect-IP: 2a02:8011:400e:2:6f00:88c8:c921:d332 X-SA-Exim-Mail-From: ben@decadent.org.uk X-SA-Exim-Scanned: No (on shadbolt.decadent.org.uk); SAEximRunCond expanded to false Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 3.16.40-rc1 review patch. If anyone has any objections, please let me know. ------------------ From: Radim Krčmář commit 2117d5398c81554fbf803f5fd1dc55eb78216c0c upstream. em_jmp_far and em_ret_far assumed that setting IP can only fail in 64 bit mode, but syzkaller proved otherwise (and SDM agrees). Code segment was restored upon failure, but it was left uninitialized outside of long mode, which could lead to a leak of host kernel stack. We could have fixed that by always saving and restoring the CS, but we take a simpler approach and just break any guest that manages to fail as the error recovery is error-prone and modern CPUs don't need emulator for this. Found by syzkaller: WARNING: CPU: 2 PID: 3668 at arch/x86/kvm/emulate.c:2217 em_ret_far+0x428/0x480 Kernel panic - not syncing: panic_on_warn set ... CPU: 2 PID: 3668 Comm: syz-executor Not tainted 4.9.0-rc4+ #49 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011 [...] Call Trace: [...] __dump_stack lib/dump_stack.c:15 [...] dump_stack+0xb3/0x118 lib/dump_stack.c:51 [...] panic+0x1b7/0x3a3 kernel/panic.c:179 [...] __warn+0x1c4/0x1e0 kernel/panic.c:542 [...] warn_slowpath_null+0x2c/0x40 kernel/panic.c:585 [...] em_ret_far+0x428/0x480 arch/x86/kvm/emulate.c:2217 [...] em_ret_far_imm+0x17/0x70 arch/x86/kvm/emulate.c:2227 [...] x86_emulate_insn+0x87a/0x3730 arch/x86/kvm/emulate.c:5294 [...] x86_emulate_instruction+0x520/0x1ba0 arch/x86/kvm/x86.c:5545 [...] emulate_instruction arch/x86/include/asm/kvm_host.h:1116 [...] complete_emulated_io arch/x86/kvm/x86.c:6870 [...] complete_emulated_mmio+0x4e9/0x710 arch/x86/kvm/x86.c:6934 [...] kvm_arch_vcpu_ioctl_run+0x3b7a/0x5a90 arch/x86/kvm/x86.c:6978 [...] kvm_vcpu_ioctl+0x61e/0xdd0 arch/x86/kvm/../../../virt/kvm/kvm_main.c:2557 [...] vfs_ioctl fs/ioctl.c:43 [...] do_vfs_ioctl+0x18c/0x1040 fs/ioctl.c:679 [...] SYSC_ioctl fs/ioctl.c:694 [...] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:685 [...] entry_SYSCALL_64_fastpath+0x1f/0xc2 Reported-by: Dmitry Vyukov Fixes: d1442d85cc30 ("KVM: x86: Handle errors when RIP is set during far jumps") Signed-off-by: Radim Krčmář [bwh: Backported to 3.16: adjust context] Signed-off-by: Ben Hutchings --- arch/x86/kvm/emulate.c | 36 +++++++++++------------------------- 1 file changed, 11 insertions(+), 25 deletions(-) --- a/arch/x86/kvm/emulate.c +++ b/arch/x86/kvm/emulate.c @@ -1981,16 +1981,10 @@ static int em_iret(struct x86_emulate_ct static int em_jmp_far(struct x86_emulate_ctxt *ctxt) { int rc; - unsigned short sel, old_sel; - struct desc_struct old_desc, new_desc; - const struct x86_emulate_ops *ops = ctxt->ops; + unsigned short sel; + struct desc_struct new_desc; 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, cpl, false, @@ -1999,12 +1993,10 @@ static int em_jmp_far(struct x86_emulate return rc; 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; - } + /* Error handling is not implemented. */ + if (rc != X86EMUL_CONTINUE) + return X86EMUL_UNHANDLEABLE; + return rc; } @@ -2070,14 +2062,8 @@ static int em_ret_far(struct x86_emulate { int rc; 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); + struct desc_struct new_desc; rc = emulate_pop(ctxt, &eip, ctxt->op_bytes); if (rc != X86EMUL_CONTINUE) @@ -2093,10 +2079,10 @@ static int em_ret_far(struct x86_emulate 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); - } + /* Error handling is not implemented. */ + if (rc != X86EMUL_CONTINUE) + return X86EMUL_UNHANDLEABLE; + return rc; }