Re: [PATCH] kvm: x86: vmx: add checks on guest RIP

[Paolo Bonzini <pbonzini@redhat.com>]

On 29/11/2014 16:27, Eugene Korenevsky wrote:
> Signed-off-by: Eugene Korenevsky <ekorenevsky@gmail.com>
> ---
> 
> Notes:
>     This patch adds checks on Guest RIP specified in Intel Software Developer Manual.
>     
>     The following checks are performed on processors that support Intel 64 architecture:
>     - Bits 63:32 must be 0 if the "IA-32e mode guest" VM-entry control is 0 or if
>     the L bit (bit 13) in the access-rights field for CS is 0.
>     - If the processor supports N < 64 linear-address bits, bits 63:N must be identical
>     if the "IA-32e mode guest" VM-entry control is 1 and the L bit in the access-rights
>     field for CS is 1. (No check applies if the processor supports 64 linear-address
>     bits.)
> 
>  arch/x86/kvm/vmx.c | 27 ++++++++++++++++++++++++++-
>  1 file changed, 26 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 6a951d8..e2da83b 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -3828,6 +3828,28 @@ static bool cs_ss_rpl_check(struct kvm_vcpu *vcpu)
>  		 (ss.selector & SELECTOR_RPL_MASK));
>  }
>  
> +#ifdef CONFIG_X86_64
> +static bool rip_valid(struct kvm_vcpu *vcpu)
> +{
> +	unsigned long rip;
> +	struct kvm_segment cs;
> +	bool longmode;
> +
> +	/* RIP must be canonical in long mode
> +	 * Bits 63:32 of RIP must be zero in other processor modes */
> +	longmode = false;
> +	if (vm_entry_controls_get(to_vmx(vcpu)) & VM_ENTRY_IA32E_MODE) {
> +		vmx_get_segment(vcpu, &cs, VCPU_SREG_CS);
> +		longmode = (cs.l != 0);
> +	}
> +	rip = kvm_register_read(vcpu, VCPU_REGS_RIP);
> +	if (longmode)
> +		return !is_noncanonical_address(rip);

This check is off by one.  It is checking bits 63:47 instead of bits
63:48 (this quirk is intentionally part of the specification, so that
you can reenter a guest at 0x800000000000 after e.g. a VMCALL vmexit and
cause a general protection fault).

However, I am not sure how this can occur.  A #GP should have been
injected as part of the instruction that caused RIP to become invalid.
Perhaps you should check in nested_vmx_run instead?

Paolo

> +	else
> +		return (rip >> 32) == 0;
> +}
> +#endif
> +
>  /*
>   * Check if guest state is valid. Returns true if valid, false if
>   * not.
> @@ -3873,8 +3895,11 @@ static bool guest_state_valid(struct kvm_vcpu *vcpu)
>  		if (!ldtr_valid(vcpu))
>  			return false;
>  	}
> +#ifdef CONFIG_X86_64
> +	if (!rip_valid(vcpu))
> +		return false;
> +#endif
>  	/* TODO:
> -	 * - Add checks on RIP
>  	 * - Add checks on RFLAGS
>  	 */
>  
> 

My