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Date: Thu, 1 Apr 2021 11:28:33 +0200
From: Emanuele Giuseppe Esposito <eesposit@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
Jonathan Corbet <corbet@....net>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Jim Mattson <jmattson@...gle.com>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
"H. Peter Anvin" <hpa@...or.com>, Shuah Khan <shuah@...nel.org>,
Alexander Graf <graf@...zon.com>,
Andrew Jones <drjones@...hat.com>, linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v2 1/4] kvm: cpuid: adjust the returned nent field of
kvm_cpuid2 for KVM_GET_SUPPORTED_CPUID and KVM_GET_EMULATED_CPUID
On 31/03/2021 20:31, Sean Christopherson wrote:
> On Wed, Mar 31, 2021, Emanuele Giuseppe Esposito wrote:
>> Calling the kvm KVM_GET_[SUPPORTED/EMULATED]_CPUID ioctl requires
>> a nent field inside the kvm_cpuid2 struct to be big enough to contain
>> all entries that will be set by kvm.
>> Therefore if the nent field is too high, kvm will adjust it to the
>> right value. If too low, -E2BIG is returned.
>>
>> However, when filling the entries do_cpuid_func() requires an
>> additional entry, so if the right nent is known in advance,
>> giving the exact number of entries won't work because it has to be
>> increased by one.
>
> I'd strong prefer to reword the shortlog and changelog. It's not immediately
> obvious what this is changing without the context from the v1 thread. E.g.
>
> KVM: x86: Fix a spurious -E2BIG in KVM_GET_EMULATED_CPUID
>
> When retrieving emulated CPUID entries, check for an insufficient array size
> if and only if KVM is actually inserting an entry. If userspace has a priori
> knowledge of the exact array size, KVM_GET_EMULATED_CPUID will incorrectly
> fail due to effectively requiring an extra, unused entry.
I will update it with v3, thanks.
>
>> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@...hat.com>
>> ---
>> arch/x86/kvm/cpuid.c | 35 ++++++++++++++++++-----------------
>> 1 file changed, 18 insertions(+), 17 deletions(-)
>>
>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>> index 6bd2f8b830e4..02a51f921548 100644
>> --- a/arch/x86/kvm/cpuid.c
>> +++ b/arch/x86/kvm/cpuid.c
>> @@ -567,34 +567,34 @@ static struct kvm_cpuid_entry2 *do_host_cpuid(struct kvm_cpuid_array *array,
>>
>> static int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func)
>> {
>> - struct kvm_cpuid_entry2 *entry;
>> -
>> - if (array->nent >= array->maxnent)
>> - return -E2BIG;
>> + struct kvm_cpuid_entry2 entry;
>>
>> - entry = &array->entries[array->nent];
>> - entry->function = func;
>> - entry->index = 0;
>> - entry->flags = 0;
>> + entry.function = func;
>> + entry.index = 0;
>> + entry.flags = 0;
>
> Depending on the leaf, eax/ebx/ecx/edx will be left uninitialized. This wasn't
> a bug before since @array is zeroed on allocation.
>
> What about pre-checking @func? I don't particular like the duplicate checks,
> but none of the solutions are particularly elegant. E.g.
You're right, I should have zeroed it. I agree that memsetting and
memcopying is not elegant either, but unless I am missing something and
it changes the intended behavior, IMHO this avoids duplicate checks and
makes it simpler to add a new 'func'.
Emanuele
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 6bd2f8b830e4..9824947bd5ad 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -565,14 +565,18 @@ static struct kvm_cpuid_entry2 *do_host_cpuid(struct kvm_cpuid_array *array,
> return entry;
> }
>
> -static int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func)
> +static noinline int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func)
> {
> struct kvm_cpuid_entry2 *entry;
>
> + if (func != 0 && func != 1 && func != 7)
> + return 0;
> +
> if (array->nent >= array->maxnent)
> return -E2BIG;
>
> - entry = &array->entries[array->nent];
> + entry = &array->entries[array->nent++];
> +
> entry->function = func;
> entry->index = 0;
> entry->flags = 0;
> @@ -580,19 +584,17 @@ static int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func)
> switch (func) {
> case 0:
> entry->eax = 7;
> - ++array->nent;
> break;
> case 1:
> entry->ecx = F(MOVBE);
> - ++array->nent;
> break;
> case 7:
> entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
> entry->eax = 0;
> entry->ecx = F(RDPID);
> - ++array->nent;
> - default:
> break;
> + default:
> + BUG();
> }
>
> return 0
>
>
>>
>> switch (func) {
>> case 0:
>> - entry->eax = 7;
>> - ++array->nent;
>> + entry.eax = 7;
>> break;
>> case 1:
>> - entry->ecx = F(MOVBE);
>> - ++array->nent;
>> + entry.ecx = F(MOVBE);
>> break;
>> case 7:
>> - entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
>> - entry->eax = 0;
>> - entry->ecx = F(RDPID);
>> - ++array->nent;
>> - default:
>> + entry.flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
>> + entry.eax = 0;
>> + entry.ecx = F(RDPID);
>> break;
>> + default:
>> + goto out;
>> }
>>
>> + if (array->nent >= array->maxnent)
>> + return -E2BIG;
>> +
>> + memcpy(&array->entries[array->nent++], &entry, sizeof(entry));
>> +
>> +out:
>> return 0;
>> }
>>
>> @@ -975,6 +975,7 @@ int kvm_dev_ioctl_get_cpuid(struct kvm_cpuid2 *cpuid,
>>
>> if (cpuid->nent < 1)
>> return -E2BIG;
>> +
>> if (cpuid->nent > KVM_MAX_CPUID_ENTRIES)
>> cpuid->nent = KVM_MAX_CPUID_ENTRIES;
>>
>> --
>> 2.30.2
>>
>
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