* Re: [PATCH] KVM: X86: eliminate some meaningless code
@ 2020-02-24 1:54 linmiaohe
0 siblings, 0 replies; 5+ messages in thread
From: linmiaohe @ 2020-02-24 1:54 UTC (permalink / raw)
To: Paolo Bonzini, Sean Christopherson
Cc: rkrcmar, vkuznets, wanpengli, jmattson, joro, tglx, mingo, bp,
hpa, kvm, linux-kernel, x86
Paolo Bonzini <pbonzini@redhat.com> wrote:
>On 21/02/20 16:23, Sean Christopherson wrote:
>>
>> I'm guessing no VMM actually uses this ioctl(), e.g. neither Qemu or
>> CrosVM use it, which is why the broken behavior has gone unnoticed.
>> Don't suppose you'd want to write a selftest to hammer KVM_{SET,GET}_CPUID2?
>>
>
>I would just drop KVM_GET_CPUID2 altogether and see if someone complains.
>
Will do. Thanks.
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] KVM: X86: eliminate some meaningless code
@ 2020-02-24 1:44 linmiaohe
0 siblings, 0 replies; 5+ messages in thread
From: linmiaohe @ 2020-02-24 1:44 UTC (permalink / raw)
To: Sean Christopherson
Cc: pbonzini, rkrcmar, vkuznets, wanpengli, jmattson, joro, tglx,
mingo, bp, hpa, kvm, linux-kernel, x86
Sean Christopherson <sean.j.christopherson@intel.com> wrote:
> On Fri, Feb 21, 2020 at 10:05:26PM +0800, linmiaohe wrote:
>> From: Miaohe Lin <linmiaohe@huawei.com>
>>
>> When kvm_vcpu_ioctl_get_cpuid2() fails, we set cpuid->nent to the
>> value of
>> vcpu->arch.cpuid_nent. But this is in vain as cpuid->nent is not
>> vcpu->copied to
>> userspace by copy_to_user() from call site. Get rid of this
>> meaningless assignment and further cleanup the var r and out jump label.
>
>Ha, took me a while to see that.
Sorry about it. I'am not good at it. :(
>>
>> On the other hand, when kvm_vcpu_ioctl_get_cpuid2() succeeds, we do
>> vcpu->arch.cpuid_nent * sizeof(struct kvm_cpuid_entry2)))
>> - goto out;
>> - return 0;
>
>Hmm, so this ioctl() is straight up broken. cpuid->nent should be updated on success so that userspace knows how many entries were retrieved, i.e.
>the code should look something like below, with kvm_arch_vcpu_ioctl() unchanged.
>
>I'm guessing no VMM actually uses this ioctl(), e.g. neither Qemu or CrosVM use it, which is why the broken behavior has gone unnoticed. Don't suppose you'd want to write a selftest to hammer KVM_{SET,GET}_CPUID2?
>
>int kvm_vcpu_ioctl_get_cpuid2(struct kvm_vcpu *vcpu,
> struct kvm_cpuid2 *cpuid,
> struct kvm_cpuid_entry2 __user *entries) {
> if (cpuid->nent < vcpu->arch.cpuid_nent)
> return -E2BIG;
>
> if (copy_to_user(entries, &vcpu->arch.cpuid_entries,
> vcpu->arch.cpuid_nent * sizeof(struct kvm_cpuid_entry2)))
> return -EFAULT;
>
> cpuid->nent = vcpu->arch.cpuid_nent;
>
> return 0;
>}
>
I searched KVM_GET_CPUID2 from Qemu, it's not used. So maybe we could just drop KVM_GET_CPUID2 altogether as
suggested by Paolo. Thanks for your review.
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] KVM: X86: eliminate some meaningless code
2020-02-21 15:23 ` Sean Christopherson
@ 2020-02-21 16:31 ` Paolo Bonzini
0 siblings, 0 replies; 5+ messages in thread
From: Paolo Bonzini @ 2020-02-21 16:31 UTC (permalink / raw)
To: Sean Christopherson, linmiaohe
Cc: rkrcmar, vkuznets, wanpengli, jmattson, joro, tglx, mingo, bp,
hpa, kvm, linux-kernel, x86
On 21/02/20 16:23, Sean Christopherson wrote:
>
> I'm guessing no VMM actually uses this ioctl(), e.g. neither Qemu or CrosVM
> use it, which is why the broken behavior has gone unnoticed. Don't suppose
> you'd want to write a selftest to hammer KVM_{SET,GET}_CPUID2?
>
> int kvm_vcpu_ioctl_get_cpuid2(struct kvm_vcpu *vcpu,
> struct kvm_cpuid2 *cpuid,
> struct kvm_cpuid_entry2 __user *entries)
> {
> if (cpuid->nent < vcpu->arch.cpuid_nent)
> return -E2BIG;
>
> if (copy_to_user(entries, &vcpu->arch.cpuid_entries,
> vcpu->arch.cpuid_nent * sizeof(struct kvm_cpuid_entry2)))
> return -EFAULT;
>
> cpuid->nent = vcpu->arch.cpuid_nent;
>
> return 0;
> }
I would just drop KVM_GET_CPUID2 altogether and see if someone complains.
Paolo
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] KVM: X86: eliminate some meaningless code
2020-02-21 14:05 linmiaohe
@ 2020-02-21 15:23 ` Sean Christopherson
2020-02-21 16:31 ` Paolo Bonzini
0 siblings, 1 reply; 5+ messages in thread
From: Sean Christopherson @ 2020-02-21 15:23 UTC (permalink / raw)
To: linmiaohe
Cc: pbonzini, rkrcmar, vkuznets, wanpengli, jmattson, joro, tglx,
mingo, bp, hpa, kvm, linux-kernel, x86
On Fri, Feb 21, 2020 at 10:05:26PM +0800, linmiaohe wrote:
> From: Miaohe Lin <linmiaohe@huawei.com>
>
> When kvm_vcpu_ioctl_get_cpuid2() fails, we set cpuid->nent to the value of
> vcpu->arch.cpuid_nent. But this is in vain as cpuid->nent is not copied to
> userspace by copy_to_user() from call site. Get rid of this meaningless
> assignment and further cleanup the var r and out jump label.
Ha, took me a while to see that.
> On the other hand, when kvm_vcpu_ioctl_get_cpuid2() succeeds, we do not
> change the content of struct cpuid. We can avoid copy_to_user() from call
> site as struct cpuid remain unchanged.
>
> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> ---
> arch/x86/kvm/cpuid.c | 14 ++++----------
> arch/x86/kvm/x86.c | 6 ------
> 2 files changed, 4 insertions(+), 16 deletions(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index b1c469446b07..b83cedc63328 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -265,20 +265,14 @@ int kvm_vcpu_ioctl_get_cpuid2(struct kvm_vcpu *vcpu,
> struct kvm_cpuid2 *cpuid,
> struct kvm_cpuid_entry2 __user *entries)
> {
> - int r;
> -
> - r = -E2BIG;
> if (cpuid->nent < vcpu->arch.cpuid_nent)
> - goto out;
> - r = -EFAULT;
> + return -E2BIG;
> +
> if (copy_to_user(entries, &vcpu->arch.cpuid_entries,
> vcpu->arch.cpuid_nent * sizeof(struct kvm_cpuid_entry2)))
> - goto out;
> - return 0;
Hmm, so this ioctl() is straight up broken. cpuid->nent should be updated
on success so that userspace knows how many entries were retrieved, i.e.
the code should look something like below, with kvm_arch_vcpu_ioctl()
unchanged.
I'm guessing no VMM actually uses this ioctl(), e.g. neither Qemu or CrosVM
use it, which is why the broken behavior has gone unnoticed. Don't suppose
you'd want to write a selftest to hammer KVM_{SET,GET}_CPUID2?
int kvm_vcpu_ioctl_get_cpuid2(struct kvm_vcpu *vcpu,
struct kvm_cpuid2 *cpuid,
struct kvm_cpuid_entry2 __user *entries)
{
if (cpuid->nent < vcpu->arch.cpuid_nent)
return -E2BIG;
if (copy_to_user(entries, &vcpu->arch.cpuid_entries,
vcpu->arch.cpuid_nent * sizeof(struct kvm_cpuid_entry2)))
return -EFAULT;
cpuid->nent = vcpu->arch.cpuid_nent;
return 0;
}
> + return -EFAULT;
>
> -out:
> - cpuid->nent = vcpu->arch.cpuid_nent;
> - return r;
> + return 0;
> }
>
> static __always_inline void cpuid_mask(u32 *word, int wordnum)
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index fbabb2f06273..683c54e7be36 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4295,12 +4295,6 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
> goto out;
> r = kvm_vcpu_ioctl_get_cpuid2(vcpu, &cpuid,
> cpuid_arg->entries);
> - if (r)
> - goto out;
> - r = -EFAULT;
> - if (copy_to_user(cpuid_arg, &cpuid, sizeof(cpuid)))
> - goto out;
> - r = 0;
> break;
> }
> case KVM_GET_MSRS: {
> --
> 2.19.1
>
^ permalink raw reply [flat|nested] 5+ messages in thread
* [PATCH] KVM: X86: eliminate some meaningless code
@ 2020-02-21 14:05 linmiaohe
2020-02-21 15:23 ` Sean Christopherson
0 siblings, 1 reply; 5+ messages in thread
From: linmiaohe @ 2020-02-21 14:05 UTC (permalink / raw)
To: pbonzini, rkrcmar, sean.j.christopherson, vkuznets, wanpengli,
jmattson, joro, tglx, mingo, bp, hpa
Cc: linmiaohe, kvm, linux-kernel, x86
From: Miaohe Lin <linmiaohe@huawei.com>
When kvm_vcpu_ioctl_get_cpuid2() fails, we set cpuid->nent to the value of
vcpu->arch.cpuid_nent. But this is in vain as cpuid->nent is not copied to
userspace by copy_to_user() from call site. Get rid of this meaningless
assignment and further cleanup the var r and out jump label.
On the other hand, when kvm_vcpu_ioctl_get_cpuid2() succeeds, we do not
change the content of struct cpuid. We can avoid copy_to_user() from call
site as struct cpuid remain unchanged.
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
---
arch/x86/kvm/cpuid.c | 14 ++++----------
arch/x86/kvm/x86.c | 6 ------
2 files changed, 4 insertions(+), 16 deletions(-)
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index b1c469446b07..b83cedc63328 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -265,20 +265,14 @@ int kvm_vcpu_ioctl_get_cpuid2(struct kvm_vcpu *vcpu,
struct kvm_cpuid2 *cpuid,
struct kvm_cpuid_entry2 __user *entries)
{
- int r;
-
- r = -E2BIG;
if (cpuid->nent < vcpu->arch.cpuid_nent)
- goto out;
- r = -EFAULT;
+ return -E2BIG;
+
if (copy_to_user(entries, &vcpu->arch.cpuid_entries,
vcpu->arch.cpuid_nent * sizeof(struct kvm_cpuid_entry2)))
- goto out;
- return 0;
+ return -EFAULT;
-out:
- cpuid->nent = vcpu->arch.cpuid_nent;
- return r;
+ return 0;
}
static __always_inline void cpuid_mask(u32 *word, int wordnum)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index fbabb2f06273..683c54e7be36 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4295,12 +4295,6 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
goto out;
r = kvm_vcpu_ioctl_get_cpuid2(vcpu, &cpuid,
cpuid_arg->entries);
- if (r)
- goto out;
- r = -EFAULT;
- if (copy_to_user(cpuid_arg, &cpuid, sizeof(cpuid)))
- goto out;
- r = 0;
break;
}
case KVM_GET_MSRS: {
--
2.19.1
^ permalink raw reply related [flat|nested] 5+ messages in thread
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2020-02-21 14:05 linmiaohe
2020-02-21 15:23 ` Sean Christopherson
2020-02-21 16:31 ` Paolo Bonzini
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