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@xxxxxxxxxx>
> ---
> 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
>
