linux-kernel - Re: [PATCH v2 05/11] KVM: s390: Add optional storage key checking to MEMOP IOCTL

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Date:   Wed, 9 Feb 2022 10:08:06 +0100
From:   Christian Borntraeger <borntraeger@...ux.ibm.com>
To:     Janis Schoetterl-Glausch <scgl@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>
Cc:     Alexander Gordeev <agordeev@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Jonathan Corbet <corbet@....net>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>
Subject: Re: [PATCH v2 05/11] KVM: s390: Add optional storage key checking to
 MEMOP IOCTL



Am 09.02.22 um 09:49 schrieb Janis Schoetterl-Glausch:
> On 2/9/22 08:34, Christian Borntraeger wrote:
>> Am 07.02.22 um 17:59 schrieb Janis Schoetterl-Glausch:
>>> User space needs a mechanism to perform key checked accesses when
>>> emulating instructions.
>>>
>>> The key can be passed as an additional argument.
>>> Having an additional argument is flexible, as user space can
>>> pass the guest PSW's key, in order to make an access the same way the
>>> CPU would, or pass another key if necessary.
>>>
>>> Signed-off-by: Janis Schoetterl-Glausch <scgl@...ux.ibm.com>
>>> Acked-by: Janosch Frank <frankja@...ux.ibm.com>
>>> Reviewed-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>
>>> ---
>>>    arch/s390/kvm/kvm-s390.c | 49 +++++++++++++++++++++++++++++++---------
>>>    include/uapi/linux/kvm.h |  8 +++++--
>>>    2 files changed, 44 insertions(+), 13 deletions(-)
>>>
>>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>>> index cf347e1a4f17..71e61fb3f0d9 100644
>>> --- a/arch/s390/kvm/kvm-s390.c
>>> +++ b/arch/s390/kvm/kvm-s390.c
>>> @@ -32,6 +32,7 @@
>>>    #include <linux/sched/signal.h>
>>>    #include <linux/string.h>
>>>    #include <linux/pgtable.h>
>>> +#include <linux/bitfield.h>
>>>      #include <asm/asm-offsets.h>
>>>    #include <asm/lowcore.h>
>>> @@ -2359,6 +2360,11 @@ static int kvm_s390_handle_pv(struct kvm *kvm, struct kvm_pv_cmd *cmd)
>>>        return r;
>>>    }
>>>    +static bool access_key_invalid(u8 access_key)
>>> +{
>>> +    return access_key > 0xf;
>>> +}
>>> +
>>>    long kvm_arch_vm_ioctl(struct file *filp,
>>>                   unsigned int ioctl, unsigned long arg)
>>>    {
>>> @@ -4687,34 +4693,54 @@ static long kvm_s390_guest_mem_op(struct kvm_vcpu *vcpu,
>>>                      struct kvm_s390_mem_op *mop)
>>>    {
>>>        void __user *uaddr = (void __user *)mop->buf;
>>> +    u8 access_key = 0, ar = 0;
>>>        void *tmpbuf = NULL;
>>> +    bool check_reserved;
>>>        int r = 0;
>>>        const u64 supported_flags = KVM_S390_MEMOP_F_INJECT_EXCEPTION
>>> -                    | KVM_S390_MEMOP_F_CHECK_ONLY;
>>> +                    | KVM_S390_MEMOP_F_CHECK_ONLY
>>> +                    | KVM_S390_MEMOP_F_SKEY_PROTECTION;
>>>    -    if (mop->flags & ~supported_flags || mop->ar >= NUM_ACRS || !mop->size)
>>> +    if (mop->flags & ~supported_flags || !mop->size)
>>>            return -EINVAL;
>>> -
>>>        if (mop->size > MEM_OP_MAX_SIZE)
>>>            return -E2BIG;
>>> -
>>>        if (kvm_s390_pv_cpu_is_protected(vcpu))
>>>            return -EINVAL;
>>> -
>>>        if (!(mop->flags & KVM_S390_MEMOP_F_CHECK_ONLY)) {
>>>            tmpbuf = vmalloc(mop->size);
>>>            if (!tmpbuf)
>>>                return -ENOMEM;
>>>        }
>>> +    ar = mop->ar;
>>> +    mop->ar = 0;
>>
>> Why this assignment to 0?
> 
> It's so the check of reserved below works like that, they're all part of the anonymous union.

Ah, I see. This is ugly :-)

>>
>>> +    if (ar >= NUM_ACRS)
>>> +        return -EINVAL;
>>> +    if (mop->flags & KVM_S390_MEMOP_F_SKEY_PROTECTION) {
>>> +        access_key = mop->key;
>>> +        mop->key = 0;
>>
>> and this? I think we can leave mop unchanged.
>>
>> In fact, why do we add the ar and access_key variable?
>> This breaks the check from above (if (mop->flags & ~supported_flags || mop->ar >= NUM_ACRS || !mop->size))  into two checks
>> and it will create a memleak for tmpbuf.
> 
> I can move the allocation down, goto out or get rid of the reserved check and keep everything as before.
> First is simpler, but second makes handling that case more explicit and might help in the future.

Maybe add a reserved_02 field in the anon struct and check this for being zero and get rid of the local variables?

> Patch 6 has the same issue in the vm ioctl handler.
>>
>> Simply use mop->key and mop->ar below and get rid of the local variables.
>> The structure has no concurrency and gcc will handle that just as the local variable.
>>
>> Other than that this looks good.
> 
> [...]
>