Re: [v3.0.y 2/2] KVM: x86: fix missing checks in syscall emulation

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Mar 22, 2012 at 09:50:44AM +0100, Stefan Bader wrote:
> >From 30870b1a5d29c07b75843c0b667fa29a63d818a4 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 8/8] 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>
> 
> (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             |   51 ++++++++++++++++++++++++++++++++++++
>  2 files changed, 64 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_emulate.h b/arch/x86/include/asm/kvm_emulate.h
> index 18e54f1..0ab6a4d 100644
> --- a/arch/x86/include/asm/kvm_emulate.h
> +++ b/arch/x86/include/asm/kvm_emulate.h
> @@ -301,6 +301,19 @@ struct x86_emulate_ctxt {
>  #define X86EMUL_MODE_PROT     (X86EMUL_MODE_PROT16|X86EMUL_MODE_PROT32| \
>  			       X86EMUL_MODE_PROT64)
>  
> +/* 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
> +
>  enum x86_intercept_stage {
>  	X86_ICTP_NONE = 0,   /* Allow zero-init to not match anything */
>  	X86_ICPT_PRE_EXCEPT,
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index adc9867..3e7d913 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -1901,6 +1901,51 @@ setup_syscalls_segments(struct x86_emulate_ctxt *ctxt,
>  	ss->p = 1;
>  }
>  
> +static bool em_syscall_is_enabled(struct x86_emulate_ctxt *ctxt)
> +{
> +	struct x86_emulate_ops *ops = ctxt->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, &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, struct x86_emulate_ops *ops)
>  {
> @@ -1915,9 +1960,15 @@ emulate_syscall(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
>  	    ctxt->mode == X86EMUL_MODE_VM86)
>  		return emulate_ud(ctxt);
>  
> +	if (!(em_syscall_is_enabled(ctxt)))
> +		return emulate_ud(ctxt);
> +
>  	ops->get_msr(ctxt, MSR_EFER, &efer);
>  	setup_syscalls_segments(ctxt, ops, &cs, &ss);
>  
> +	if (!(efer & EFER_SCE))
> +		return emulate_ud(ctxt);
> +
>  	ops->get_msr(ctxt, MSR_STAR, &msr_data);
>  	msr_data >>= 32;
>  	cs_sel = (u16)(msr_data & 0xfffc);

ACK