From: Sean Christopherson <seanjc@google.com>
To: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Cc: kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
Jonathan Corbet <corbet@lwn.net>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Jim Mattson <jmattson@google.com>, Ingo Molnar <mingo@redhat.com>,
Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
Shuah Khan <shuah@kernel.org>, Alexander Graf <graf@amazon.com>,
Andrew Jones <drjones@redhat.com>,
linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-kselftest@vger.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
Date: Wed, 31 Mar 2021 18:31:10 +0000 [thread overview]
Message-ID: <YGS/7hQo91cVuXbB@google.com> (raw)
In-Reply-To: <20210331122649.38323-2-eesposit@redhat.com>
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.
> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.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.
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
>
next prev parent reply other threads:[~2021-03-31 18:32 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-31 12:26 [PATCH 0/4] kvm: cpuid: fix cpuid nent field Emanuele Giuseppe Esposito
2021-03-31 12:26 ` [PATCH v2 1/4] kvm: cpuid: adjust the returned nent field of kvm_cpuid2 for KVM_GET_SUPPORTED_CPUID and KVM_GET_EMULATED_CPUID Emanuele Giuseppe Esposito
2021-03-31 18:31 ` Sean Christopherson [this message]
2021-04-01 9:28 ` Emanuele Giuseppe Esposito
2021-03-31 12:26 ` [PATCH v2 2/4] Documentation: kvm: update KVM_GET_EMULATED_CPUID ioctl description Emanuele Giuseppe Esposito
2021-03-31 12:26 ` [PATCH v2 3/4] selftests: add kvm_get_emulated_cpuid Emanuele Giuseppe Esposito
2021-03-31 12:26 ` [PATCH v2 4/4] selftests: kvm: add get_emulated_cpuid test Emanuele Giuseppe Esposito
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