linux-kernel - KVM_SET_CPUID doesn't check supported bits (was Re: [PATCH 0/6] KVM: x86: KVM_SET

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Message-ID: <b38dff0b-7e6d-3f3e-9724-8e280938628a@yandex.ru>
Date:   Mon, 7 Dec 2020 14:24:58 +0300
From:   stsp <stsp2@...dex.ru>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: KVM_SET_CPUID doesn't check supported bits (was Re: [PATCH 0/6] KVM:
 x86: KVM_SET_SREGS.CR4 bug fixes and cleanup)

[re-send because of bad formatting]

09.10.2020 18:30, Sean Christopherson пишет:
>> The only other effect of setting VMXE was clearing VME. Which shouldn't
>> affect anything either, right?
> Hmm, clearing VME would mean that exceptions/interrupts within the 
> guest would
> trigger a switch out of v86 and into vanilla protected mode. v86 and 
> PM have
> different consistency checks, particularly for segmentation, so it's 
> plausible
> that clearing CR4.VME inadvertantly worked around the bug by avoiding 
> invalid
> guest state for v86.

Almost.

So with your patch set (thanks!) and a
bit of further investigations, it now became
clear where the problem is.
We have this code:
---

|cpuid->nent = 2; // Use the same values as in emu-i386/simx86/interp.c 
// (Pentium 133-200MHz, "GenuineIntel") cpuid->entries[0] = (struct 
kvm_cpuid_entry) { .function = 0, .eax = 1, .ebx = 0x756e6547, .ecx = 
0x6c65746e, .edx = 0x49656e69 }; // family 5, model 2, stepping 12, fpu 
vme de pse tsc msr mce cx8 cpuid->entries[1] = (struct kvm_cpuid_entry) 
{ .function = 1, .eax = 0x052c, .ebx = 0, .ecx = 0, .edx = 0x1bf }; ret 
= ioctl(vcpufd, KVM_SET_CPUID, cpuid); free(cpuid); if (ret == -1) { 
perror("KVM: KVM_SET_CPUID"); return 0; }|

---


It tries to enable VME among other things.
qemu appears to disable VME by default,
unless you do "-cpu host". So we have a situation where
the host (which is qemu) doesn't have VME,
and guest (dosemu) is trying to enable it.
Now obviously KVM_SET_CPUID doesn't check anyting
at all and returns success. That later turns
into an invalid guest state.


Question: should KVM_SET_CPUID check for
supported bits, end return error if not everything
is supported?