Patchwork [Maverick,2/2] KVM: x86: fix missing checks in syscall emulation

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Submitter Stefan Bader
Date March 14, 2012, 10:18 p.m.
Message ID <1331763526-32743-5-git-send-email-stefan.bader@canonical.com>
Download mbox | patch
Permalink /patch/146742/
State New
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Comments

Stefan Bader - March 14, 2012, 10:18 p.m.
From b754f2a594c7de2743cbafd0d943fbbef93d1ddf Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Stephan=20B=C3=A4rwolf?= <stephan.baerwolf@tu-ilmenau.de>
Date: Thu, 12 Jan 2012 16:43:04 +0100
Subject: [PATCH 4/4] KVM: x86: fix missing checks in syscall emulation

On hosts without this patch, 32bit guests will crash (and 64bit guests
may behave in a wrong way) for example by simply executing following
nasm-demo-application:

    [bits 32]
    global _start
    SECTION .text
    _start: syscall

(I tested it with winxp and linux - both always crashed)

    Disassembly of section .text:

    00000000 <_start>:
       0:   0f 05                   syscall

The reason seems a missing "invalid opcode"-trap (int6) for the
syscall opcode "0f05", which is not available on Intel CPUs
within non-longmodes, as also on some AMD CPUs within legacy-mode.
(depending on CPU vendor, MSR_EFER and cpuid)

Because previous mentioned OSs may not engage corresponding
syscall target-registers (STAR, LSTAR, CSTAR), they remain
NULL and (non trapping) syscalls are leading to multiple
faults and finally crashs.

Depending on the architecture (AMD or Intel) pretended by
guests, various checks according to vendor's documentation
are implemented to overcome the current issue and behave
like the CPUs physical counterparts.

[mtosatti: cleanup/beautify code]

Signed-off-by: Stephan Baerwolf <stephan.baerwolf@tu-ilmenau.de>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

CVE-2012-0045
BugLink: http://bugs.launchpad.net/bugs/917842
(backported from commit c2226fc9e87ba3da060e47333657cd6616652b84 upstream)
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
---
 arch/x86/include/asm/kvm_emulate.h |   13 ++++++++
 arch/x86/kvm/emulate.c             |   61 ++++++++++++++++++++++++++++++++++-
 2 files changed, 72 insertions(+), 2 deletions(-)

Patch

diff --git a/arch/x86/include/asm/kvm_emulate.h b/arch/x86/include/asm/kvm_emulate.h
index c1be828..4cdfb12 100644
--- a/arch/x86/include/asm/kvm_emulate.h
+++ b/arch/x86/include/asm/kvm_emulate.h
@@ -228,6 +228,19 @@  struct x86_emulate_ctxt {
 #define X86EMUL_MODE_PROT32   4	/* 32-bit protected mode. */
 #define X86EMUL_MODE_PROT64   8	/* 64-bit (long) mode.    */
 
+/* CPUID vendors */
+#define X86EMUL_CPUID_VENDOR_AuthenticAMD_ebx 0x68747541
+#define X86EMUL_CPUID_VENDOR_AuthenticAMD_ecx 0x444d4163
+#define X86EMUL_CPUID_VENDOR_AuthenticAMD_edx 0x69746e65
+
+#define X86EMUL_CPUID_VENDOR_AMDisbetterI_ebx 0x69444d41
+#define X86EMUL_CPUID_VENDOR_AMDisbetterI_ecx 0x21726574
+#define X86EMUL_CPUID_VENDOR_AMDisbetterI_edx 0x74656273
+
+#define X86EMUL_CPUID_VENDOR_GenuineIntel_ebx 0x756e6547
+#define X86EMUL_CPUID_VENDOR_GenuineIntel_ecx 0x6c65746e
+#define X86EMUL_CPUID_VENDOR_GenuineIntel_edx 0x49656e69
+
 /* Host execution mode. */
 #if defined(CONFIG_X86_32)
 #define X86EMUL_MODE_HOST X86EMUL_MODE_PROT32
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 582c8fc..cb29021 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -1842,12 +1842,58 @@  setup_syscalls_segments(struct x86_emulate_ctxt *ctxt,
 	ss->present = 1;
 }
 
+static bool syscall_is_enabled(struct x86_emulate_ctxt *ctxt,
+			       struct x86_emulate_ops *ops)
+{
+	u32 eax, ebx, ecx, edx;
+
+	/*
+	 * syscall should always be enabled in longmode - so only become
+	 * vendor specific (cpuid) if other modes are active...
+	 */
+	if (ctxt->mode == X86EMUL_MODE_PROT64)
+		return true;
+
+	eax = 0x00000000;
+	ecx = 0x00000000;
+	if (ops->get_cpuid(ctxt->vcpu, &eax, &ebx, &ecx, &edx)) {
+		/*
+		 * Intel ("GenuineIntel")
+		 * remark: Intel CPUs only support "syscall" in 64bit
+		 * longmode. Also an 64bit guest with a
+		 * 32bit compat-app running will #UD !! While this
+		 * behaviour can be fixed (by emulating) into AMD
+		 * response - CPUs of AMD can't behave like Intel.
+		 */
+		if (ebx == X86EMUL_CPUID_VENDOR_GenuineIntel_ebx &&
+		    ecx == X86EMUL_CPUID_VENDOR_GenuineIntel_ecx &&
+		    edx == X86EMUL_CPUID_VENDOR_GenuineIntel_edx)
+			return false;
+
+		/* AMD ("AuthenticAMD") */
+		if (ebx == X86EMUL_CPUID_VENDOR_AuthenticAMD_ebx &&
+		    ecx == X86EMUL_CPUID_VENDOR_AuthenticAMD_ecx &&
+		    edx == X86EMUL_CPUID_VENDOR_AuthenticAMD_edx)
+			return true;
+
+		/* AMD ("AMDisbetter!") */
+		if (ebx == X86EMUL_CPUID_VENDOR_AMDisbetterI_ebx &&
+		    ecx == X86EMUL_CPUID_VENDOR_AMDisbetterI_ecx &&
+		    edx == X86EMUL_CPUID_VENDOR_AMDisbetterI_edx)
+			return true;
+	}
+
+	/* default: (not Intel, not AMD), apply Intel's stricter rules... */
+	return false;
+}
+
 static int
-emulate_syscall(struct x86_emulate_ctxt *ctxt)
+emulate_syscall(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
 {
 	struct decode_cache *c = &ctxt->decode;
 	struct kvm_segment cs, ss;
 	u64 msr_data;
+	u64 efer = 0;
 
 	/* syscall is not available in real mode */
 	if (ctxt->mode == X86EMUL_MODE_REAL ||
@@ -1856,8 +1902,19 @@  emulate_syscall(struct x86_emulate_ctxt *ctxt)
 		return X86EMUL_PROPAGATE_FAULT;
 	}
 
+	if (!(syscall_is_enabled(ctxt, ops))) {
+		kvm_queue_exception(ctxt->vcpu, UD_VECTOR);
+		return X86EMUL_PROPAGATE_FAULT;
+	}
+
+	kvm_x86_ops->get_msr(ctxt->vcpu, MSR_EFER, &efer);
 	setup_syscalls_segments(ctxt, &cs, &ss);
 
+	if (!(efer & EFER_SCE)) {
+		kvm_queue_exception(ctxt->vcpu, UD_VECTOR);
+		return X86EMUL_PROPAGATE_FAULT;
+	}
+
 	kvm_x86_ops->get_msr(ctxt->vcpu, MSR_STAR, &msr_data);
 	msr_data >>= 32;
 	cs.selector = (u16)(msr_data & 0xfffc);
@@ -3062,7 +3119,7 @@  twobyte_insn:
 		}
 		break;
 	case 0x05: 		/* syscall */
-		rc = emulate_syscall(ctxt);
+		rc = emulate_syscall(ctxt, ops);
 		if (rc != X86EMUL_CONTINUE)
 			goto done;
 		else