Re: [RFC PATCH v1 02/10] KVM: s390: Honor storage keys when accessing guest memory

[Christian Borntraeger <borntraeger@linux.ibm.com>]

Am 20.01.22 um 09:58 schrieb Janis Schoetterl-Glausch:
> On 1/20/22 09:50, Christian Borntraeger wrote:
>>
>>
>> Am 20.01.22 um 09:11 schrieb Janis Schoetterl-Glausch:
>>> On 1/19/22 20:27, Christian Borntraeger wrote:
>>>> Am 18.01.22 um 10:52 schrieb Janis Schoetterl-Glausch:
>>>>> Storage key checking had not been implemented for instructions emulated
>>>>> by KVM. Implement it by enhancing the functions used for guest access,
>>>>> in particular those making use of access_guest which has been renamed
>>>>> to access_guest_with_key.
>>>>> Accesses via access_guest_real should not be key checked.
>>>>>
>>>>> For actual accesses, key checking is done by __copy_from/to_user_with_key
>>>>> (which internally uses MVCOS/MVCP/MVCS).
>>>>> In cases where accessibility is checked without an actual access,
>>>>> this is performed by getting the storage key and checking
>>>>> if the access key matches.
>>>>> In both cases, if applicable, storage and fetch protection override
>>>>> are honored.
>>>>>
>>>>> Signed-off-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
>>>>> ---
>>>>>     arch/s390/include/asm/ctl_reg.h |   2 +
>>>>>     arch/s390/include/asm/page.h    |   2 +
>>>>>     arch/s390/kvm/gaccess.c         | 174 +++++++++++++++++++++++++++++---
>>>>>     arch/s390/kvm/gaccess.h         |  78 ++++++++++++--
>>>>>     arch/s390/kvm/intercept.c       |  12 +--
>>>>>     arch/s390/kvm/kvm-s390.c        |   4 +-
>>>>>     6 files changed, 241 insertions(+), 31 deletions(-)
>>>>>
>>>>> diff --git a/arch/s390/include/asm/ctl_reg.h b/arch/s390/include/asm/ctl_reg.h
>>>>> index 04dc65f8901d..c800199a376b 100644
>>>>> --- a/arch/s390/include/asm/ctl_reg.h
>>>>> +++ b/arch/s390/include/asm/ctl_reg.h
>>>>> @@ -12,6 +12,8 @@
>>>>>       #define CR0_CLOCK_COMPARATOR_SIGN    BIT(63 - 10)
>>>>>     #define CR0_LOW_ADDRESS_PROTECTION    BIT(63 - 35)
>>>>> +#define CR0_FETCH_PROTECTION_OVERRIDE    BIT(63 - 38)
>>>>> +#define CR0_STORAGE_PROTECTION_OVERRIDE    BIT(63 - 39)
>>>>>     #define CR0_EMERGENCY_SIGNAL_SUBMASK    BIT(63 - 49)
>>>>>     #define CR0_EXTERNAL_CALL_SUBMASK    BIT(63 - 50)
>>>>>     #define CR0_CLOCK_COMPARATOR_SUBMASK    BIT(63 - 52)
>>>>> diff --git a/arch/s390/include/asm/page.h b/arch/s390/include/asm/page.h
>>>>> index d98d17a36c7b..cfc4d6fb2385 100644
>>>>> --- a/arch/s390/include/asm/page.h
>>>>> +++ b/arch/s390/include/asm/page.h
>>>>> @@ -20,6 +20,8 @@
>>>>>     #define PAGE_SIZE    _PAGE_SIZE
>>>>>     #define PAGE_MASK    _PAGE_MASK
>>>>>     #define PAGE_DEFAULT_ACC    0
>>>>> +/* storage-protection override */
>>>>> +#define PAGE_SPO_ACC        9
>>>>>     #define PAGE_DEFAULT_KEY    (PAGE_DEFAULT_ACC << 4)
>>>>>       #define HPAGE_SHIFT    20
>>>>> diff --git a/arch/s390/kvm/gaccess.c b/arch/s390/kvm/gaccess.c
>>>>> index 4460808c3b9a..92ab96d55504 100644
>>>>> --- a/arch/s390/kvm/gaccess.c
>>>>> +++ b/arch/s390/kvm/gaccess.c
>>>>> @@ -10,6 +10,7 @@
>>>>>     #include <linux/mm_types.h>
>>>>>     #include <linux/err.h>
>>>>>     #include <linux/pgtable.h>
>>>>> +#include <linux/bitfield.h>
>>>>>       #include <asm/gmap.h>
>>>>>     #include "kvm-s390.h"
>>>>> @@ -794,6 +795,79 @@ static int low_address_protection_enabled(struct kvm_vcpu *vcpu,
>>>>>         return 1;
>>>>>     }
>>>>>     +static bool fetch_prot_override_applicable(struct kvm_vcpu *vcpu, enum gacc_mode mode,
>>>>> +                       union asce asce)
>>>>> +{
>>>>> +    psw_t *psw = &vcpu->arch.sie_block->gpsw;
>>>>> +    unsigned long override;
>>>>> +
>>>>> +    if (mode == GACC_FETCH || mode == GACC_IFETCH) {
>>>>> +        /* check if fetch protection override enabled */
>>>>> +        override = vcpu->arch.sie_block->gcr[0];
>>>>> +        override &= CR0_FETCH_PROTECTION_OVERRIDE;
>>>>> +        /* not applicable if subject to DAT && private space */
>>>>> +        override = override && !(psw_bits(*psw).dat && asce.p);
>>>>> +        return override;
>>>>> +    }
>>>>> +    return false;
>>>>> +}
>>>>> +
>>>>> +static bool fetch_prot_override_applies(unsigned long ga, unsigned int len)
>>>>> +{
>>>>> +    return ga < 2048 && ga + len <= 2048;
>>>>> +}
>>>>> +
>>>>> +static bool storage_prot_override_applicable(struct kvm_vcpu *vcpu)
>>>>> +{
>>>>> +    /* check if storage protection override enabled */
>>>>> +    return vcpu->arch.sie_block->gcr[0] & CR0_STORAGE_PROTECTION_OVERRIDE;
>>>>> +}
>>>>> +
>>>>> +static bool storage_prot_override_applies(char access_control)
>>>>> +{
>>>>> +    /* matches special storage protection override key (9) -> allow */
>>>>> +    return access_control == PAGE_SPO_ACC;
>>>>> +}
>>>>> +
>>>>> +static int vcpu_check_access_key(struct kvm_vcpu *vcpu, char access_key,
>>>>> +                 enum gacc_mode mode, union asce asce, gpa_t gpa,
>>>>> +                 unsigned long ga, unsigned int len)
>>>>> +{
>>>>> +    unsigned char storage_key, access_control;
>>>>> +    unsigned long hva;
>>>>> +    int r;
>>>>> +
>>>>> +    /* access key 0 matches any storage key -> allow */
>>>>> +    if (access_key == 0)
>>>>> +        return 0;
>>>>> +    /*
>>>>> +     * caller needs to ensure that gfn is accessible, so we can
>>>>> +     * assume that this cannot fail
>>>>> +     */
>>>>> +    hva = gfn_to_hva(vcpu->kvm, gpa_to_gfn(gpa));
>>>>> +    mmap_read_lock(current->mm);
>>>>> +    r = get_guest_storage_key(current->mm, hva, &storage_key);
>>>>> +    mmap_read_unlock(current->mm);
>>>>> +    if (r)
>>>>> +        return r;
>>>>> +    access_control = FIELD_GET(_PAGE_ACC_BITS, storage_key);
>>>>> +    /* access key matches storage key -> allow */
>>>>> +    if (access_control == access_key)
>>>>> +        return 0;
>>>>> +    if (mode == GACC_FETCH || mode == GACC_IFETCH) {
>>>>> +        /* mismatching keys, no fetch protection -> allowed */
>>>>> +        if (!(storage_key & _PAGE_FP_BIT))
>>>>> +            return 0;
>>>>> +        if (fetch_prot_override_applicable(vcpu, mode, asce))
>>>>> +            if (fetch_prot_override_applies(ga, len))
>>>>> +                return 0;
>>>>> +    }
>>>>> +    if (storage_prot_override_applicable(vcpu))
>>>>> +        if (storage_prot_override_applies(access_control))
>>>>> +            return 0;
>>>>> +    return PGM_PROTECTION;
>>>>> +}
>>>>
>>>> This function is just a pre-check (and early-exit) and we do an additional final check
>>>> in the MVCOS routing later on, correct? It might actually be faster to get rid of this
>>>
>>> No, this exists for those cases that do not do an actual access, that is MEMOPs with
>>> the check only flag, as well as the TEST PROTECTION emulation. access_guest_with_key
>>> passes key 0 so we take the early return. It's easy to miss so Janosch suggested a comment there.
>>
>> Dont we always call it in guest_range_to_gpas, which is also called for the memory access in
>> access_guest_with_key?
> @@ -904,16 +1018,37 @@ int access_guest(struct kvm_vcpu *vcpu, unsigned long ga, u8 ar, void *data,
>   		gpas = vmalloc(array_size(nr_pages, sizeof(unsigned long)));
>   	if (!gpas)
>   		return -ENOMEM;
> +	try_fetch_prot_override = fetch_prot_override_applicable(vcpu, mode, asce);
> +	try_storage_prot_override = storage_prot_override_applicable(vcpu);
>   	need_ipte_lock = psw_bits(*psw).dat && !asce.r;
>   	if (need_ipte_lock)
>   		ipte_lock(vcpu);
> -	rc = guest_range_to_gpas(vcpu, ga, ar, gpas, len, asce, mode);
> -	for (idx = 0; idx < nr_pages && !rc; idx++) {
> +	rc = guest_range_to_gpas(vcpu, ga, ar, gpas, len, asce, mode, 0);
> 
> Yes, but the key is 0 in that case, so we don't do any key checking.  ^

So yes, we need a comment :-)