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;
entry->ecx = F(MOVBE);++array->nent;
entry.ecx = F(MOVBE);
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);- 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