Re: [PATCH for 4.4-stable] KVM: x86: fix singlestepping over syscall

[David Hildenbrand]

On 16.11.2017 19:07, Paolo Bonzini wrote:
> [ Upstream commit c8401dda2f0a00cd25c0af6a95ed50e478d25de4 ]
> 
> TF is handled a bit differently for syscall and sysret, compared
> to the other instructions: TF is checked after the instruction completes,
> so that the OS can disable #DB at a syscall by adding TF to FMASK.
> When the sysret is executed the #DB is taken "as if" the syscall insn
> just completed.
> 
> KVM emulates syscall so that it can trap 32-bit syscall on Intel processors.
> Fix the behavior, otherwise you could get #DB on a user stack which is not
> nice.  This does not affect Linux guests, as they use an IST or task gate
> for #DB.
> 
> This fixes CVE-2017-7518.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: Andy Lutomirski <luto@xxxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Signed-off-by: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> 
> Conflicts:
> 	arch/x86/kvm/x86.c
> ---
>  arch/x86/include/asm/kvm_emulate.h |  1 +
>  arch/x86/kvm/emulate.c             |  1 +
>  arch/x86/kvm/x86.c                 | 52 ++++++++++++++++----------------------
>  3 files changed, 24 insertions(+), 30 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_emulate.h b/arch/x86/include/asm/kvm_emulate.h
> index 19d14ac23ef9..fc3c7e49c8e4 100644
> --- a/arch/x86/include/asm/kvm_emulate.h
> +++ b/arch/x86/include/asm/kvm_emulate.h
> @@ -296,6 +296,7 @@ struct x86_emulate_ctxt {
>  
>  	bool perm_ok; /* do not check permissions if true */
>  	bool ud;	/* inject an #UD if host doesn't support insn */
> +	bool tf;	/* TF value before instruction (after for syscall/sysret) */
>  
>  	bool have_exception;
>  	struct x86_exception exception;
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 04b2f3cad7ba..684edebb4a0c 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -2726,6 +2726,7 @@ static int em_syscall(struct x86_emulate_ctxt *ctxt)
>  		ctxt->eflags &= ~(X86_EFLAGS_VM | X86_EFLAGS_IF);
>  	}
>  
> +	ctxt->tf = (ctxt->eflags & X86_EFLAGS_TF) != 0;
>  	return X86EMUL_CONTINUE;
>  }
>  
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 8e526c6fd784..3ffd5900da5b 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5095,6 +5095,8 @@ static void init_emulate_ctxt(struct kvm_vcpu *vcpu)
>  	kvm_x86_ops->get_cs_db_l_bits(vcpu, &cs_db, &cs_l);
>  
>  	ctxt->eflags = kvm_get_rflags(vcpu);
> +	ctxt->tf = (ctxt->eflags & X86_EFLAGS_TF) != 0;
> +
>  	ctxt->eip = kvm_rip_read(vcpu);
>  	ctxt->mode = (!is_protmode(vcpu))		? X86EMUL_MODE_REAL :
>  		     (ctxt->eflags & X86_EFLAGS_VM)	? X86EMUL_MODE_VM86 :
> @@ -5315,37 +5317,26 @@ static int kvm_vcpu_check_hw_bp(unsigned long addr, u32 type, u32 dr7,
>  	return dr6;
>  }
>  
> -static void kvm_vcpu_check_singlestep(struct kvm_vcpu *vcpu, unsigned long rflags, int *r)
> +static void kvm_vcpu_do_singlestep(struct kvm_vcpu *vcpu, int *r)
>  {
>  	struct kvm_run *kvm_run = vcpu->run;
>  
> -	/*
> -	 * rflags is the old, "raw" value of the flags.  The new value has
> -	 * not been saved yet.
> -	 *
> -	 * This is correct even for TF set by the guest, because "the
> -	 * processor will not generate this exception after the instruction
> -	 * that sets the TF flag".
> -	 */
> -	if (unlikely(rflags & X86_EFLAGS_TF)) {
> -		if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
> -			kvm_run->debug.arch.dr6 = DR6_BS | DR6_FIXED_1 |
> -						  DR6_RTM;
> -			kvm_run->debug.arch.pc = vcpu->arch.singlestep_rip;
> -			kvm_run->debug.arch.exception = DB_VECTOR;
> -			kvm_run->exit_reason = KVM_EXIT_DEBUG;
> -			*r = EMULATE_USER_EXIT;
> -		} else {
> -			vcpu->arch.emulate_ctxt.eflags &= ~X86_EFLAGS_TF;
> -			/*
> -			 * "Certain debug exceptions may clear bit 0-3.  The
> -			 * remaining contents of the DR6 register are never
> -			 * cleared by the processor".
> -			 */
> -			vcpu->arch.dr6 &= ~15;
> -			vcpu->arch.dr6 |= DR6_BS | DR6_RTM;
> -			kvm_queue_exception(vcpu, DB_VECTOR);
> -		}
> +	if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
> +		kvm_run->debug.arch.dr6 = DR6_BS | DR6_FIXED_1 | DR6_RTM;
> +		kvm_run->debug.arch.pc = vcpu->arch.singlestep_rip;
> +		kvm_run->debug.arch.exception = DB_VECTOR;
> +		kvm_run->exit_reason = KVM_EXIT_DEBUG;
> +		*r = EMULATE_USER_EXIT;
> +	} else {
> +		vcpu->arch.emulate_ctxt.eflags &= ~X86_EFLAGS_TF;
> +		/*
> +		 * "Certain debug exceptions may clear bit 0-3.  The
> +		 * remaining contents of the DR6 register are never
> +		 * cleared by the processor".
> +		 */
> +		vcpu->arch.dr6 &= ~15;
> +		vcpu->arch.dr6 |= DR6_BS | DR6_RTM;
> +		kvm_queue_exception(vcpu, DB_VECTOR);
>  	}
>  }

This looks good to me.

General question: how do we treat KVM single stepping and concurrent TF
in the guest? IOW, shouldn't the "else" rather be a check for ctxt->tf ?
(or is that handled later on e.g. in user space?)

But question unrelated to this patch, so

Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>

>  
> @@ -5500,8 +5491,9 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
>  		toggle_interruptibility(vcpu, ctxt->interruptibility);
>  		vcpu->arch.emulate_regs_need_sync_to_vcpu = false;
>  		kvm_rip_write(vcpu, ctxt->eip);
> -		if (r == EMULATE_DONE)
> -			kvm_vcpu_check_singlestep(vcpu, rflags, &r);
> +		if (r == EMULATE_DONE &&
> +		    (ctxt->tf || (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)))
> +			kvm_vcpu_do_singlestep(vcpu, &r);
>  		if (!ctxt->have_exception ||
>  		    exception_type(ctxt->exception.vector) == EXCPT_TRAP)
>  			__kvm_set_rflags(vcpu, ctxt->eflags);
> 


-- 

Thanks,

David / dhildenb