linux-kernel - Re: [PATCH 6/7] KVM: arm64: Change return type of kvm_vm_ioctl_mte_copy

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Message-ID: <ce11001e-dbe5-8694-71a5-841f4d614456@arm.com>
Date:   Wed, 8 Feb 2023 11:51:30 +0000
From:   Steven Price <steven.price@....com>
To:     Cornelia Huck <cohuck@...hat.com>, Gavin Shan <gshan@...hat.com>,
        Thomas Huth <thuth@...hat.com>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     kvmarm@...ts.linux.dev, linux-kernel@...r.kernel.org,
        kvm-riscv@...ts.infradead.org, Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Zenghui Yu <yuzenghui@...wei.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        linuxppc-dev@...ts.ozlabs.org, Eric Auger <eric.auger@...hat.com>
Subject: Re: [PATCH 6/7] KVM: arm64: Change return type of
 kvm_vm_ioctl_mte_copy_tags() to "int"

On 08/02/2023 08:49, Cornelia Huck wrote:
> On Wed, Feb 08 2023, Gavin Shan <gshan@...hat.com> wrote:
> 
>> On 2/7/23 9:09 PM, Thomas Huth wrote:
>>> Oh, drat, I thought I had checked all return statements ... this must have fallen through the cracks, sorry!
>>>
>>> Anyway, this is already a problem now: The function is called from kvm_arch_vm_ioctl() (which still returns a long), which in turn is called from kvm_vm_ioctl() in virt/kvm/kvm_main.c. And that functions stores the return value in an "int r" variable. So the upper bits are already lost there.

Sorry about that, I was caught out by kvm_arch_vm_ioctl() returning long...

>>> Also, how is this supposed to work from user space? The normal "ioctl()" libc function just returns an "int" ? Is this ioctl already used in a userspace application somewhere? ... at least in QEMU, I didn't spot it yet...
>>>
> 
> We will need it in QEMU to implement migration with MTE (the current
> proposal simply adds a migration blocker when MTE is enabled, as there
> are various other things that need to be figured out for this to work.)
> But maybe other VMMs already use it (and have been lucky because they
> always dealt with shorter lengths?)
> 
>>
>> The ioctl command KVM_ARM_MTE_COPY_TAGS was merged recently and not used
>> by QEMU yet. I think struct kvm_arm_copy_mte_tags::length needs to be
>> '__u32' instead of '__u64' in order to standardize the return value.
>> Something like below. Documentation/virt/kvm/api.rst::section-4.130
>> needs update accordingly.
>>
>>     struct kvm_arm_copy_mte_tags {
>>          __u64 guest_ipa;
>>          __u32 pad;
>>          __u32 length;
>>          void __user *addr;
>>          __u64 flags;
>>          __u64 reserved[2];
>>    };
> 
> Can we do this in a more compatible way, as we are dealing with an API?
> Like returning -EINVAL if length is too big?
> 

I agree the simplest fix for the problem is simply to reject any
lengths>INT_MAX:

diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
index cf4c495a4321..94aed7ce85c4 100644
--- a/arch/arm64/kvm/guest.c
+++ b/arch/arm64/kvm/guest.c
@@ -1032,6 +1032,13 @@ long kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
 	if (copy_tags->flags & ~KVM_ARM_TAGS_FROM_GUEST)
 		return -EINVAL;

+	/*
+	 * ioctl returns int, so lengths above INT_MAX cannot be
+	 * represented in the return value
+	 */
+	if (length > INT_MAX)
+		return -EINVAL;
+
 	if (length & ~PAGE_MASK || guest_ipa & ~PAGE_MASK)
 		return -EINVAL;

This could also be fixed in a useable way by including a new flag which
returns the length in an output field of the ioctl structure. I'm
guessing a 2GB limit would be annoying to work around.

Steve