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

[Andy Lutomirski <luto@amacapital.net>]

On 12/01/2014 08:27 AM, Paolo Bonzini wrote:
> 
> 
> 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).

Seriously?  Intel did that for vmcall but not sysret?

> 
> 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?

For syscall/sysret, at least, if you put a syscall at the highest
possible non-negative canonical address, the sysret will fault on the
way back.

--Andy

> 
> 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
> --
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