Re: [PATCH 2/5] KVM: s390: Add support for machine checks.

From: Christian Borntraeger <borntraeger@de.ibm.com>
To: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Cornelia Huck <cornelia.huck@de.ibm.com>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	Gleb Natapov <gleb@redhat.com>, KVM <kvm@vger.kernel.org>,
	linux-s390 <linux-s390@vger.kernel.org>,
	Avi Kivity <avi.kivity@gmail.com>,
	Carsten Otte <cotte@de.ibm.com>, Alexander Graf <agraf@suse.de>,
	Martin Schwidefsky <schwidefsky@de.ibm.com>,
	Sebastian Ott <sebott@linux.vnet.ibm.com>
Subject: Re: [PATCH 2/5] KVM: s390: Add support for machine checks.
Date: Wed, 19 Dec 2012 11:20:18 +0100	[thread overview]
Message-ID: <50D194E2.8050102@de.ibm.com> (raw)
In-Reply-To: <20121219094414.GA4996@osiris.de.ibm.com>

On 19/12/12 10:44, Heiko Carstens wrote:
> On Fri, Dec 07, 2012 at 01:30:22PM +0100, Cornelia Huck wrote:
>> +		rc = put_guest_u64(vcpu, __LC_MCCK_CODE, inti->mchk.mcic);
>> +		if (rc == -EFAULT)
>> +			exception = 1;
>> +
>> +		rc = copy_to_guest(vcpu, __LC_MCK_OLD_PSW,
>> +				   &vcpu->arch.sie_block->gpsw, sizeof(psw_t));
>> +		if (rc == -EFAULT)
>> +			exception = 1;
> 
> Please don't add more explicit -EFAULT checks on guest access paths. Just
> make this like normal user space accesses. That is return code != 0 means
> an error occured:
> 
> 	rc = put_guest_u64(vcpu, __LC_MCCK_CODE, inti->mchk.mcic);
> 	if (rc)
> 		exception = 1;
> 
> In fact, with the current kvm gaccess code it's even broken, since on error
> the guest access functions may return also -ENOMEM instead of -EFAULT, which
> would be ignored by your code.
> I addressed that with a patch when trying to clean up the guest access
> functions. Maybe the patch below should be merged anyway. Christian?

The whole guest memory access of KVM needs to be reworked to work properly
in those corner cases. I have this on my todo list as one of things for next
year with lots of open questions that I dont want to answer before xmas.
what about in kernel intercepts? (shall we then return EFAULT for the KVM_RUN
ioctl, shall we kill the guest?.....)

We actually need to test the address for validity via the memslots (and not
via return value of copy_from/to_user) all across the s390 code.

I really want to avoid mixing this effort with the virtio-ccw patches.
So my proposal is to apply your patch below and keep Conny's patch as is.
Ok?

Christian

> From db05454b6f3f49a7a10f5a1e546917303cf94532 Mon Sep 17 00:00:00 2001
> From: Heiko Carstens <heiko.carstens@de.ibm.com>
> Date: Mon, 10 Sep 2012 16:36:23 +0200
> Subject: s390/kvm,gaccess: fix guest access return code handling
> 
> Guest access functions like copy_to/from_guest() call __guestaddr_to_user()
> which in turn call gmap_fault() in order to translate a guest address to a
> user space address.
> In error case __guest_addr_to_user() returns either -EFAULT or -ENOMEM.
> The copy_to/from_guest functions just pass these return values down to the
> callers.
> The -ENOMEM case however is problematic since there are several places
> which access guest memory like:
> 
> rc = copy_to_guest(...);
> if (rc == -EFAULT)
> 	error_handling();
> 
> So in case of -ENOMEM the code assumes that the guest memory access
> succeeded even though it failed.
> This can cause guest data or state corruption.
> 
> If __guestaddr_to_user() returns -ENOMEM the meaning is that a valid user
> space mapping exists, but there was not enough memory available when trying
> to build the guest mapping. In other words an out-of-memory situation
> occured.
> For normal user space accesses an out-of-memory situation causes the page
> fault handler to map -ENOMEM to -EFAULT (see fixup code in do_no_context()).
> We need to do exactly the same for the kvm gaccess functions.
> 
> So __guestaddr_to_user() should just map all error codes to -EFAULT.
> 
> Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>

> ---
>  arch/s390/kvm/gaccess.h | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/s390/kvm/gaccess.h b/arch/s390/kvm/gaccess.h
> index 4703f12..84d01dd 100644
> --- a/arch/s390/kvm/gaccess.h
> +++ b/arch/s390/kvm/gaccess.h
> @@ -22,13 +22,16 @@ static inline void __user *__guestaddr_to_user(struct kvm_vcpu *vcpu,
>  					       unsigned long guestaddr)
>  {
>  	unsigned long prefix  = vcpu->arch.sie_block->prefix;
> +	unsigned long uaddress;
> 
>  	if (guestaddr < 2 * PAGE_SIZE)
>  		guestaddr += prefix;
>  	else if ((guestaddr >= prefix) && (guestaddr < prefix + 2 * PAGE_SIZE))
>  		guestaddr -= prefix;
> -
> -	return (void __user *) gmap_fault(guestaddr, vcpu->arch.gmap);
> +	uaddress = gmap_fault(guestaddr, vcpu->arch.gmap);
> +	if (IS_ERR_VALUE(uaddress))
> +		uaddress = -EFAULT;
> +	return (void __user *)uaddress;
>  }
> 
>  static inline int get_guest_u64(struct kvm_vcpu *vcpu, unsigned long guestaddr,
> 