[PATCH v2 0/2] KVM: arm64: Assorted IPA size fixes

[PATCH v2 0/2] KVM: arm64: Assorted IPA size fixes

[Marc Zyngier]

This is a rework of an initial patch posted a couple of days back[1]

While working on enabling KVM on "reduced IPA size" systems, I realise
we have a couple of issues, some of while do impact userspace.

The first issue is that we accept the creation of a "default IPA size"
VM (40 bits) even when the HW doesn't support it. Not good.

The second one is that we disallow a memslot to end right where the
IPA limit is. One page less and you're good, but that's not quite what
it should be.

I intend for both patches to be backported to -stable.

Thanks,

	M.

* From v1:
  - Don't try to cap the default IPA size. If userspace uses 0 with an
    expectation that it will get 40bits, we should abide by it and
    return an error immediately (noticed by Andrew)
  - Added a new patch to fix the exclusive nature of the IPA limit
  
[1] https://lore.kernel.org/r/20210308174643.761100-1-maz@kernel.org

Marc Zyngier (2):
  KVM: arm64: Reject VM creation when the default IPA size is
    unsupported
  KVM: arm64: Fix exclusive limit for IPA size

 Documentation/virt/kvm/api.rst |  3 +++
 arch/arm64/kvm/mmu.c           |  3 +--
 arch/arm64/kvm/reset.c         | 12 ++++++++----
 3 files changed, 12 insertions(+), 6 deletions(-)

-- 
2.29.2


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[PATCH v2 1/2] KVM: arm64: Reject VM creation when the default IPA size is unsupported

[Marc Zyngier]

KVM/arm64 has forever used a 40bit default IPA space, partially
due to its 32bit heritage (where the only choice is 40bit).

However, there are implementations in the wild that have a *cough*
much smaller *cough* IPA space, which leads to a misprogramming of
VTCR_EL2, and a guest that is stuck on its first memory access
if userspace dares to ask for the default IPA setting (which most
VMMs do).

Instead, blundly reject the creation of such VM, as we can't
satisfy the requirements from userspace (with a one-off warning).
Also clarify the boot warning, and document that the VM creation
will fail when an unsupported IPA size is probided.

Although this is an ABI change, it doesn't really change much
for userspace:

- the guest couldn't run before this change, but no error was
  returned. At least userspace knows what is happening.

- a memory slot that was accepted because it did fit the default
  IPA space now doesn't even get a chance to be registered.

The other thing that is left doing is to convince userspace to
actually use the IPA space setting instead of relying on the
antiquated default.

Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 Documentation/virt/kvm/api.rst |  3 +++
 arch/arm64/kvm/reset.c         | 12 ++++++++----
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 1a2b5210cdbf..38e327d4b479 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -182,6 +182,9 @@ is dependent on the CPU capability and the kernel configuration. The limit can
 be retrieved using KVM_CAP_ARM_VM_IPA_SIZE of the KVM_CHECK_EXTENSION
 ioctl() at run-time.
 
+Creation of the VM will fail if the requested IPA size (whether it is
+implicit or explicit) is unsupported on the host.
+
 Please note that configuring the IPA size does not affect the capability
 exposed by the guest CPUs in ID_AA64MMFR0_EL1[PARange]. It only affects
 size of the address translated by the stage2 level (guest physical to
diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
index 47f3f035f3ea..f1a38405934e 100644
--- a/arch/arm64/kvm/reset.c
+++ b/arch/arm64/kvm/reset.c
@@ -324,10 +324,9 @@ int kvm_set_ipa_limit(void)
 	}
 
 	kvm_ipa_limit = id_aa64mmfr0_parange_to_phys_shift(parange);
-	WARN(kvm_ipa_limit < KVM_PHYS_SHIFT,
-	     "KVM IPA Size Limit (%d bits) is smaller than default size\n",
-	     kvm_ipa_limit);
-	kvm_info("IPA Size Limit: %d bits\n", kvm_ipa_limit);
+	kvm_info("IPA Size Limit: %d bits%s\n", kvm_ipa_limit,
+		 ((kvm_ipa_limit < KVM_PHYS_SHIFT) ?
+		  " (Reduced IPA size, limited VM/VMM compatibility)" : ""));
 
 	return 0;
 }
@@ -356,6 +355,11 @@ int kvm_arm_setup_stage2(struct kvm *kvm, unsigned long type)
 			return -EINVAL;
 	} else {
 		phys_shift = KVM_PHYS_SHIFT;
+		if (phys_shift > kvm_ipa_limit) {
+			pr_warn_once("%s using unsupported default IPA limit, upgrade your VMM\n",
+				     current->comm, kvm_ipa_limit);
+			return -EINVAL;
+		}
 	}
 
 	mmfr0 = read_sanitised_ftr_reg(SYS_ID_AA64MMFR0_EL1);
-- 
2.29.2


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[PATCH v2 2/2] KVM: arm64: Fix exclusive limit for IPA size

[Marc Zyngier]

When registering a memslot, we check the size and location of that
memslot against the IPA size to ensure that we can provide guest
access to the whole of the memory.

Unfortunately, this check rejects memslot that end-up at the exact
limit of the addressing capability for a given IPA size. For example,
it refuses the creatrion of a 2GB memslot at 0x8000000 with a 32bit
IPA space.

Fix it by relaxing the check to accept a memslot reaching the
limit of the IPA space.

Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/arm64/kvm/mmu.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 77cb2d28f2a4..8711894db8c2 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1312,8 +1312,7 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
 	 * Prevent userspace from creating a memory region outside of the IPA
 	 * space addressable by the KVM guest IPA space.
 	 */
-	if (memslot->base_gfn + memslot->npages >=
-	    (kvm_phys_size(kvm) >> PAGE_SHIFT))
+	if ((memslot->base_gfn + memslot->npages) > (kvm_phys_size(kvm) >> PAGE_SHIFT))
 		return -EFAULT;
 
 	mmap_read_lock(current->mm);
-- 
2.29.2


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Re: [PATCH v2 2/2] KVM: arm64: Fix exclusive limit for IPA size

[Auger Eric]

Hi Marc,

On 3/10/21 11:42 AM, Marc Zyngier wrote:
> When registering a memslot, we check the size and location of that
> memslot against the IPA size to ensure that we can provide guest
> access to the whole of the memory.
> 
> Unfortunately, this check rejects memslot that end-up at the exact
> limit of the addressing capability for a given IPA size. For example,
> it refuses the creatrion of a 2GB memslot at 0x8000000 with a 32bit
creation
> IPA space.
> 
> Fix it by relaxing the check to accept a memslot reaching the
> limit of the IPA space.
> 
> Signed-off-by: Marc Zyngier <maz@kernel.org>
stable + Fixes?
> ---
>  arch/arm64/kvm/mmu.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 77cb2d28f2a4..8711894db8c2 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1312,8 +1312,7 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
>  	 * Prevent userspace from creating a memory region outside of the IPA
>  	 * space addressable by the KVM guest IPA space.
>  	 */
> -	if (memslot->base_gfn + memslot->npages >=
> -	    (kvm_phys_size(kvm) >> PAGE_SHIFT))
> +	if ((memslot->base_gfn + memslot->npages) > (kvm_phys_size(kvm) >> PAGE_SHIFT))
>  		return -EFAULT;
>  
>  	mmap_read_lock(current->mm);
> 
Reviewed-by: Eric Auger <eric.auger@redhat.com>

Thanks

Eric


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Re: [PATCH v2 1/2] KVM: arm64: Reject VM creation when the default IPA size is unsupported

[Auger Eric]

Hi Marc,

On 3/10/21 11:42 AM, Marc Zyngier wrote:
> KVM/arm64 has forever used a 40bit default IPA space, partially
> due to its 32bit heritage (where the only choice is 40bit).
> 
> However, there are implementations in the wild that have a *cough*
> much smaller *cough* IPA space, which leads to a misprogramming of
> VTCR_EL2, and a guest that is stuck on its first memory access
> if userspace dares to ask for the default IPA setting (which most
> VMMs do).
> 
> Instead, blundly reject the creation of such VM, as we can't
> satisfy the requirements from userspace (with a one-off warning).
> Also clarify the boot warning, and document that the VM creation
> will fail when an unsupported IPA size is probided.
> 
> Although this is an ABI change, it doesn't really change much
> for userspace:
> 
> - the guest couldn't run before this change, but no error was
>   returned. At least userspace knows what is happening.
> 
> - a memory slot that was accepted because it did fit the default
>   IPA space now doesn't even get a chance to be registered.
> 
> The other thing that is left doing is to convince userspace to
> actually use the IPA space setting instead of relying on the
> antiquated default.
> 
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  Documentation/virt/kvm/api.rst |  3 +++
>  arch/arm64/kvm/reset.c         | 12 ++++++++----
>  2 files changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 1a2b5210cdbf..38e327d4b479 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -182,6 +182,9 @@ is dependent on the CPU capability and the kernel configuration. The limit can
>  be retrieved using KVM_CAP_ARM_VM_IPA_SIZE of the KVM_CHECK_EXTENSION
>  ioctl() at run-time.
>  
> +Creation of the VM will fail if the requested IPA size (whether it is
> +implicit or explicit) is unsupported on the host.
> +
>  Please note that configuring the IPA size does not affect the capability
>  exposed by the guest CPUs in ID_AA64MMFR0_EL1[PARange]. It only affects
>  size of the address translated by the stage2 level (guest physical to
> diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
> index 47f3f035f3ea..f1a38405934e 100644
> --- a/arch/arm64/kvm/reset.c
> +++ b/arch/arm64/kvm/reset.c
> @@ -324,10 +324,9 @@ int kvm_set_ipa_limit(void)
>  	}
>  
>  	kvm_ipa_limit = id_aa64mmfr0_parange_to_phys_shift(parange);
> -	WARN(kvm_ipa_limit < KVM_PHYS_SHIFT,
> -	     "KVM IPA Size Limit (%d bits) is smaller than default size\n",
> -	     kvm_ipa_limit);
> -	kvm_info("IPA Size Limit: %d bits\n", kvm_ipa_limit);
> +	kvm_info("IPA Size Limit: %d bits%s\n", kvm_ipa_limit,
> +		 ((kvm_ipa_limit < KVM_PHYS_SHIFT) ?
> +		  " (Reduced IPA size, limited VM/VMM compatibility)" : ""));
>  
>  	return 0;
>  }
> @@ -356,6 +355,11 @@ int kvm_arm_setup_stage2(struct kvm *kvm, unsigned long type)
>  			return -EINVAL;
>  	} else {
>  		phys_shift = KVM_PHYS_SHIFT;
> +		if (phys_shift > kvm_ipa_limit) {
> +			pr_warn_once("%s using unsupported default IPA limit, upgrade your VMM\n",
> +				     current->comm, kvm_ipa_limit);
don't you have a trouble with the args here?

Otherwise looks sensible to me.

Thanks

Eric
> +			return -EINVAL;
> +		}
>  	}
>  
>  	mmfr0 = read_sanitised_ftr_reg(SYS_ID_AA64MMFR0_EL1);
> 


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Re: [PATCH v2 1/2] KVM: arm64: Reject VM creation when the default IPA size is unsupported

[Marc Zyngier]

Hi Eric,

On 2021-03-10 10:59, Auger Eric wrote:
> Hi Marc,
> 
> On 3/10/21 11:42 AM, Marc Zyngier wrote:
>> KVM/arm64 has forever used a 40bit default IPA space, partially
>> due to its 32bit heritage (where the only choice is 40bit).
>> 
>> However, there are implementations in the wild that have a *cough*
>> much smaller *cough* IPA space, which leads to a misprogramming of
>> VTCR_EL2, and a guest that is stuck on its first memory access
>> if userspace dares to ask for the default IPA setting (which most
>> VMMs do).
>> 
>> Instead, blundly reject the creation of such VM, as we can't
>> satisfy the requirements from userspace (with a one-off warning).
>> Also clarify the boot warning, and document that the VM creation
>> will fail when an unsupported IPA size is probided.
>> 
>> Although this is an ABI change, it doesn't really change much
>> for userspace:
>> 
>> - the guest couldn't run before this change, but no error was
>>   returned. At least userspace knows what is happening.
>> 
>> - a memory slot that was accepted because it did fit the default
>>   IPA space now doesn't even get a chance to be registered.
>> 
>> The other thing that is left doing is to convince userspace to
>> actually use the IPA space setting instead of relying on the
>> antiquated default.
>> 
>> Signed-off-by: Marc Zyngier <maz@kernel.org>
>> ---
>>  Documentation/virt/kvm/api.rst |  3 +++
>>  arch/arm64/kvm/reset.c         | 12 ++++++++----
>>  2 files changed, 11 insertions(+), 4 deletions(-)
>> 
>> diff --git a/Documentation/virt/kvm/api.rst 
>> b/Documentation/virt/kvm/api.rst
>> index 1a2b5210cdbf..38e327d4b479 100644
>> --- a/Documentation/virt/kvm/api.rst
>> +++ b/Documentation/virt/kvm/api.rst
>> @@ -182,6 +182,9 @@ is dependent on the CPU capability and the kernel 
>> configuration. The limit can
>>  be retrieved using KVM_CAP_ARM_VM_IPA_SIZE of the KVM_CHECK_EXTENSION
>>  ioctl() at run-time.
>> 
>> +Creation of the VM will fail if the requested IPA size (whether it is
>> +implicit or explicit) is unsupported on the host.
>> +
>>  Please note that configuring the IPA size does not affect the 
>> capability
>>  exposed by the guest CPUs in ID_AA64MMFR0_EL1[PARange]. It only 
>> affects
>>  size of the address translated by the stage2 level (guest physical to
>> diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
>> index 47f3f035f3ea..f1a38405934e 100644
>> --- a/arch/arm64/kvm/reset.c
>> +++ b/arch/arm64/kvm/reset.c
>> @@ -324,10 +324,9 @@ int kvm_set_ipa_limit(void)
>>  	}
>> 
>>  	kvm_ipa_limit = id_aa64mmfr0_parange_to_phys_shift(parange);
>> -	WARN(kvm_ipa_limit < KVM_PHYS_SHIFT,
>> -	     "KVM IPA Size Limit (%d bits) is smaller than default size\n",
>> -	     kvm_ipa_limit);
>> -	kvm_info("IPA Size Limit: %d bits\n", kvm_ipa_limit);
>> +	kvm_info("IPA Size Limit: %d bits%s\n", kvm_ipa_limit,
>> +		 ((kvm_ipa_limit < KVM_PHYS_SHIFT) ?
>> +		  " (Reduced IPA size, limited VM/VMM compatibility)" : ""));
>> 
>>  	return 0;
>>  }
>> @@ -356,6 +355,11 @@ int kvm_arm_setup_stage2(struct kvm *kvm, 
>> unsigned long type)
>>  			return -EINVAL;
>>  	} else {
>>  		phys_shift = KVM_PHYS_SHIFT;
>> +		if (phys_shift > kvm_ipa_limit) {
>> +			pr_warn_once("%s using unsupported default IPA limit, upgrade your 
>> VMM\n",
>> +				     current->comm, kvm_ipa_limit);
> don't you have a trouble with the args here?
> 
> Otherwise looks sensible to me.

This is what happens when you do a last minute change and push it out
without thinking.

My apologies, I'll fix that.

         M.
-- 
Jazz is not dead. It just smells funny...

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