[PATCH] KVM: x86: fix CPUID entries returned by KVM_GET_CPUID2 ioctl

[PATCH] KVM: x86: fix CPUID entries returned by KVM_GET_CPUID2 ioctl

[Michael Roth]

Recent commit 255cbecfe0 modified struct kvm_vcpu_arch to make
'cpuid_entries' a pointer to an array of kvm_cpuid_entry2 entries
rather than embedding the array in the struct. KVM_SET_CPUID and
KVM_SET_CPUID2 were updated accordingly, but KVM_GET_CPUID2 was missed.

As a result, KVM_GET_CPUID2 currently returns random fields from struct
kvm_vcpu_arch to userspace rather than the expected CPUID values. Fix
this by treating 'cpuid_entries' as a pointer when copying its
contents to userspace buffer.

Fixes: 255cbecfe0c9 ("KVM: x86: allocate vcpu->arch.cpuid_entries dynamically")
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Michael Roth <michael.roth@amd.com.com>
---
 arch/x86/kvm/cpuid.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 13036cf0b912..38172ca627d3 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -321,7 +321,7 @@ int kvm_vcpu_ioctl_get_cpuid2(struct kvm_vcpu *vcpu,
 	if (cpuid->nent < vcpu->arch.cpuid_nent)
 		goto out;
 	r = -EFAULT;
-	if (copy_to_user(entries, &vcpu->arch.cpuid_entries,
+	if (copy_to_user(entries, vcpu->arch.cpuid_entries,
 			 vcpu->arch.cpuid_nent * sizeof(struct kvm_cpuid_entry2)))
 		goto out;
 	return 0;
-- 
2.25.1

Re: [PATCH] KVM: x86: fix CPUID entries returned by KVM_GET_CPUID2 ioctl

[Paolo Bonzini]

On 28/01/21 03:44, Michael Roth wrote:
> Recent commit 255cbecfe0 modified struct kvm_vcpu_arch to make
> 'cpuid_entries' a pointer to an array of kvm_cpuid_entry2 entries
> rather than embedding the array in the struct. KVM_SET_CPUID and
> KVM_SET_CPUID2 were updated accordingly, but KVM_GET_CPUID2 was missed.
> 
> As a result, KVM_GET_CPUID2 currently returns random fields from struct
> kvm_vcpu_arch to userspace rather than the expected CPUID values. Fix
> this by treating 'cpuid_entries' as a pointer when copying its
> contents to userspace buffer.
> 
> Fixes: 255cbecfe0c9 ("KVM: x86: allocate vcpu->arch.cpuid_entries dynamically")
> Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
> Signed-off-by: Michael Roth <michael.roth@amd.com.com>
> ---
>   arch/x86/kvm/cpuid.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 13036cf0b912..38172ca627d3 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -321,7 +321,7 @@ int kvm_vcpu_ioctl_get_cpuid2(struct kvm_vcpu *vcpu,
>   	if (cpuid->nent < vcpu->arch.cpuid_nent)
>   		goto out;
>   	r = -EFAULT;
> -	if (copy_to_user(entries, &vcpu->arch.cpuid_entries,
> +	if (copy_to_user(entries, vcpu->arch.cpuid_entries,
>   			 vcpu->arch.cpuid_nent * sizeof(struct kvm_cpuid_entry2)))
>   		goto out;
>   	return 0;
> 

Queued, thanks.  I'll also write a testcase.

Paolo

Re: [PATCH] KVM: x86: fix CPUID entries returned by KVM_GET_CPUID2 ioctl

[Vitaly Kuznetsov]

Michael Roth <michael.roth@amd.com> writes:

> Recent commit 255cbecfe0 modified struct kvm_vcpu_arch to make
> 'cpuid_entries' a pointer to an array of kvm_cpuid_entry2 entries
> rather than embedding the array in the struct. KVM_SET_CPUID and
> KVM_SET_CPUID2 were updated accordingly, but KVM_GET_CPUID2 was missed.
>
> As a result, KVM_GET_CPUID2 currently returns random fields from struct
> kvm_vcpu_arch to userspace rather than the expected CPUID values. Fix
> this by treating 'cpuid_entries' as a pointer when copying its
> contents to userspace buffer.
>
> Fixes: 255cbecfe0c9 ("KVM: x86: allocate vcpu->arch.cpuid_entries dynamically")
> Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
> Signed-off-by: Michael Roth <michael.roth@amd.com.com>
> ---
>  arch/x86/kvm/cpuid.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 13036cf0b912..38172ca627d3 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -321,7 +321,7 @@ int kvm_vcpu_ioctl_get_cpuid2(struct kvm_vcpu *vcpu,
>  	if (cpuid->nent < vcpu->arch.cpuid_nent)
>  		goto out;
>  	r = -EFAULT;
> -	if (copy_to_user(entries, &vcpu->arch.cpuid_entries,
> +	if (copy_to_user(entries, vcpu->arch.cpuid_entries,
>  			 vcpu->arch.cpuid_nent * sizeof(struct kvm_cpuid_entry2)))
>  		goto out;
>  	return 0;

This is embarrassing but I have a (possible) excuse: copy_to_user's
argument is 'void *' so no warning was produced. Surprisingly, no test
caught the breakage. Thanks for debugging and fixing!

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

-- 
Vitaly

Re: [PATCH] KVM: x86: fix CPUID entries returned by KVM_GET_CPUID2 ioctl

[Paolo Bonzini]

On 28/01/21 09:36, Vitaly Kuznetsov wrote:
> This is embarrassing but I have a (possible) excuse: copy_to_user's
> argument is 'void *' so no warning was produced. Surprisingly, no test
> caught the breakage. Thanks for debugging and fixing!

So who writes the test:

- the author of the buggy patch

- the maintainer who failed to spot it

- the poor sod who fixed the issue

?

(Just kidding, it will of course be either me or a fourth person :)).

Paolo