Messages in this thread | | | Date | Wed, 8 Feb 2023 13:16:38 +0100 | From | Thomas Huth <> | Subject | Re: [PATCH 6/7] KVM: arm64: Change return type of kvm_vm_ioctl_mte_copy_tags() to "int" |
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On 08/02/2023 12.51, Steven Price wrote: > On 08/02/2023 08:49, Cornelia Huck wrote: >> On Wed, Feb 08 2023, Gavin Shan <gshan@redhat.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...
That's why I'm trying to fix that return type mess with my series, to avoid such problems in the future :-)
>>>> 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.
I agree that checking for length > INT_MAX is likely the best thing to do here right now. I'll add that in v2 of my series.
But actually, this might even be a good thing from another point of view (so I'm not sure whether your idea with the flag should really be pursued): The code here takes a mutex and then runs a while loop that depends on the length - which could cause the lock to be held for a rather long time if length is a 64-bit value. Forcing the user space to limit the length here could help to avoid taking the lock for too long.
Thomas
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