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Message-ID: <3e50722f-0b96-78ce-6d5a-6b6f9931f218@linux.ibm.com>
Date: Thu, 20 Jan 2022 09:50:09 +0100
From: Christian Borntraeger <borntraeger@...ux.ibm.com>
To: Janis Schoetterl-Glausch <scgl@...ux.ibm.com>,
Heiko Carstens <hca@...ux.ibm.com>,
Vasily Gorbik <gor@...ux.ibm.com>,
Janosch Frank <frankja@...ux.ibm.com>,
Alexander Gordeev <agordeev@...ux.ibm.com>,
David Hildenbrand <david@...hat.com>
Cc: Claudio Imbrenda <imbrenda@...ux.ibm.com>,
linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
kvm@...r.kernel.org
Subject: Re: [RFC PATCH v1 02/10] KVM: s390: Honor storage keys when accessing
guest memory
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@...ux.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?
>
>> pre-test and simply rely on MVCOS. MVCOS is usually just some cycles while ISKE to read
>> the key is really slow path and take hundreds of cycles. This would even simplify the
>> patch (assuming that we do proper key checking all the time).
>
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