linux-kernel - Re: [PATCH v2] KVM: s390: Test for bad access register and size at the start of S390_MEM

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Date:   Thu, 29 Aug 2019 15:38:34 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Cornelia Huck <cohuck@...hat.com>, Thomas Huth <thuth@...hat.com>
Cc:     Janosch Frank <frankja@...ux.ibm.com>, kvm@...r.kernel.org,
        David Hildenbrand <david@...hat.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] KVM: s390: Test for bad access register and size at
 the start of S390_MEM_OP



On 29.08.19 14:40, Cornelia Huck wrote:
> On Thu, 29 Aug 2019 14:25:17 +0200
> Thomas Huth <thuth@...hat.com> wrote:
> 
>> If the KVM_S390_MEM_OP ioctl is called with an access register >= 16,
>> then there is certainly a bug in the calling userspace application.
>> We check for wrong access registers, but only if the vCPU was already
>> in the access register mode before (i.e. the SIE block has recorded
>> it). The check is also buried somewhere deep in the calling chain (in
>> the function ar_translation()), so this is somewhat hard to find.
>>
>> It's better to always report an error to the userspace in case this
>> field is set wrong, and it's safer in the KVM code if we block wrong
>> values here early instead of relying on a check somewhere deep down
>> the calling chain, so let's add another check to kvm_s390_guest_mem_op()
>> directly.
>>
>> We also should check that the "size" is non-zero here (thanks to Janosch
>> Frank for the hint!). If we do not check the size, we could call vmalloc()
>> with this 0 value, and this will cause a kernel warning.
>>
>> Signed-off-by: Thomas Huth <thuth@...hat.com>

I will add cc stable.

Thanks applied.
>> ---
>>  v2: Check mop->size to be non-zero
>>
>>  arch/s390/kvm/kvm-s390.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>> index f329dcb3f44c..49d7722229ae 100644
>> --- a/arch/s390/kvm/kvm-s390.c
>> +++ b/arch/s390/kvm/kvm-s390.c
>> @@ -4255,7 +4255,7 @@ static long kvm_s390_guest_mem_op(struct kvm_vcpu *vcpu,
>>  	const u64 supported_flags = KVM_S390_MEMOP_F_INJECT_EXCEPTION
>>  				    | KVM_S390_MEMOP_F_CHECK_ONLY;
>>  
>> -	if (mop->flags & ~supported_flags)
>> +	if (mop->flags & ~supported_flags || mop->ar >= NUM_ACRS || !mop->size)
>>  		return -EINVAL;
>>  
>>  	if (mop->size > MEM_OP_MAX_SIZE)
> 
> Reviewed-by: Cornelia Huck <cohuck@...hat.com>
>