Re: [RFC] KVM: x86: Support KVM VMs sharing SEV context
From: Steve Rutherford
Date: Thu Feb 25 2021 - 13:51:40 EST
On Thu, Feb 25, 2021 at 6:57 AM Tom Lendacky <thomas.lendacky@xxxxxxx> wrote:
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>> +int svm_vm_copy_asid_to(struct kvm *kvm, unsigned int mirror_kvm_fd)
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>> +{
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>> + struct file *mirror_kvm_file;
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>> + struct kvm *mirror_kvm;
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>> + struct kvm_sev_info *mirror_kvm_sev;
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>> + unsigned int asid;
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>> + int ret;
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>> +
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>> + if (!sev_guest(kvm))
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>> + return -ENOTTY;
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>
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> You definitely don't want this: this is the function that turns the vm
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> into an SEV guest (marks SEV as active).
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>
The sev_guest() function does not set sev->active, it only checks it. The
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sev_guest_init() function is where sev->active is set.
Sorry, bad use of the english on my part: the "this" was referring to
svm_vm_copy_asid_to. Right now, you could only pass this sev_guest
check if you had already called sev_guest_init, which seems incorrect.
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>
>
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> (Not an issue with this patch, but a broader issue) I believe
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> sev_guest lacks the necessary acquire/release barriers on sev->active,
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>
The svm_mem_enc_op() takes the kvm lock and that is the only way into the
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sev_guest_init() function where sev->active is set.
There are a few places that check sev->active which don't have the kvm
lock, which is not problematic if we add in a few compiler barriers
(ala irqchip_split et al).
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>
Thanks,
>
Tom
Thanks,
Steve