[PATCH] KVM: SVM: no need to call access_ok() in LAUNCH_MEASURE command

[PATCH] KVM: SVM: no need to call access_ok() in LAUNCH_MEASURE command

[Brijesh Singh]

Using the access_ok() to validate the input before issuing the SEV
command does not buy us anything in this case. If userland is
giving us a garbage pointer then copy_to_user() will catch it when we try
to return the measurement.

Suggested-by: Al Viro <viro@ZenIV.linux.org.uk>
Fixes: 0d0736f76347 (KVM: SVM: Add support for KVM_SEV_LAUNCH_MEASURE ...)
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Radim Krčmář" <rkrcmar@redhat.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: linux-kernel@vger.kernel.org
Cc: Joerg Roedel <joro@8bytes.org>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
---

We no longer need patch [1]. This patch implements Al Viro's recommendation [2]

[1] https://marc.info/?l=linux-kernel&m=151905677729098&w=2.
[2] https://marc.info/?l=linux-kernel&m=151923536116467&w=2

 arch/x86/kvm/svm.c | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index b3e488a74828..ca69d53d7e6d 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -6236,16 +6236,18 @@ static int sev_launch_update_data(struct kvm *kvm, struct kvm_sev_cmd *argp)
 
 static int sev_launch_measure(struct kvm *kvm, struct kvm_sev_cmd *argp)
 {
+	void __user *measure = (void __user *)(uintptr_t)argp->data;
 	struct kvm_sev_info *sev = &kvm->arch.sev_info;
 	struct sev_data_launch_measure *data;
 	struct kvm_sev_launch_measure params;
+	void __user *p = NULL;
 	void *blob = NULL;
 	int ret;
 
 	if (!sev_guest(kvm))
 		return -ENOTTY;
 
-	if (copy_from_user(&params, (void __user *)(uintptr_t)argp->data, sizeof(params)))
+	if (copy_from_user(&params, measure, sizeof(params)))
 		return -EFAULT;
 
 	data = kzalloc(sizeof(*data), GFP_KERNEL);
@@ -6256,17 +6258,13 @@ static int sev_launch_measure(struct kvm *kvm, struct kvm_sev_cmd *argp)
 	if (!params.len)
 		goto cmd;
 
-	if (params.uaddr) {
+	p = (void __user *)(uintptr_t)params.uaddr;
+	if (p) {
 		if (params.len > SEV_FW_BLOB_MAX_SIZE) {
 			ret = -EINVAL;
 			goto e_free;
 		}
 
-		if (!access_ok(VERIFY_WRITE, params.uaddr, params.len)) {
-			ret = -EFAULT;
-			goto e_free;
-		}
-
 		ret = -ENOMEM;
 		blob = kmalloc(params.len, GFP_KERNEL);
 		if (!blob)
@@ -6290,13 +6288,13 @@ static int sev_launch_measure(struct kvm *kvm, struct kvm_sev_cmd *argp)
 		goto e_free_blob;
 
 	if (blob) {
-		if (copy_to_user((void __user *)(uintptr_t)params.uaddr, blob, params.len))
+		if (copy_to_user(p, blob, params.len))
 			ret = -EFAULT;
 	}
 
 done:
 	params.len = data->len;
-	if (copy_to_user((void __user *)(uintptr_t)argp->data, &params, sizeof(params)))
+	if (copy_to_user(measure, &params, sizeof(params)))
 		ret = -EFAULT;
 e_free_blob:
 	kfree(blob);
-- 
2.14.3

Re: [PATCH] KVM: SVM: no need to call access_ok() in LAUNCH_MEASURE command

[Paolo Bonzini]

On 23/02/2018 19:36, Brijesh Singh wrote:
> Using the access_ok() to validate the input before issuing the SEV
> command does not buy us anything in this case. If userland is
> giving us a garbage pointer then copy_to_user() will catch it when we try
> to return the measurement.
> 
> Suggested-by: Al Viro <viro@ZenIV.linux.org.uk>
> Fixes: 0d0736f76347 (KVM: SVM: Add support for KVM_SEV_LAUNCH_MEASURE ...)
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: "Radim Krčmář" <rkrcmar@redhat.com>
> Cc: Borislav Petkov <bp@suse.de>
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Cc: linux-kernel@vger.kernel.org
> Cc: Joerg Roedel <joro@8bytes.org>
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> ---
> 
> We no longer need patch [1]. This patch implements Al Viro's recommendation [2]
> 
> [1] https://marc.info/?l=linux-kernel&m=151905677729098&w=2.
> [2] https://marc.info/?l=linux-kernel&m=151923536116467&w=2
> 
>  arch/x86/kvm/svm.c | 16 +++++++---------
>  1 file changed, 7 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index b3e488a74828..ca69d53d7e6d 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -6236,16 +6236,18 @@ static int sev_launch_update_data(struct kvm *kvm, struct kvm_sev_cmd *argp)
>  
>  static int sev_launch_measure(struct kvm *kvm, struct kvm_sev_cmd *argp)
>  {
> +	void __user *measure = (void __user *)(uintptr_t)argp->data;
>  	struct kvm_sev_info *sev = &kvm->arch.sev_info;
>  	struct sev_data_launch_measure *data;
>  	struct kvm_sev_launch_measure params;
> +	void __user *p = NULL;
>  	void *blob = NULL;
>  	int ret;
>  
>  	if (!sev_guest(kvm))
>  		return -ENOTTY;
>  
> -	if (copy_from_user(&params, (void __user *)(uintptr_t)argp->data, sizeof(params)))
> +	if (copy_from_user(&params, measure, sizeof(params)))
>  		return -EFAULT;
>  
>  	data = kzalloc(sizeof(*data), GFP_KERNEL);
> @@ -6256,17 +6258,13 @@ static int sev_launch_measure(struct kvm *kvm, struct kvm_sev_cmd *argp)
>  	if (!params.len)
>  		goto cmd;
>  
> -	if (params.uaddr) {
> +	p = (void __user *)(uintptr_t)params.uaddr;
> +	if (p) {
>  		if (params.len > SEV_FW_BLOB_MAX_SIZE) {
>  			ret = -EINVAL;
>  			goto e_free;
>  		}
>  
> -		if (!access_ok(VERIFY_WRITE, params.uaddr, params.len)) {
> -			ret = -EFAULT;
> -			goto e_free;
> -		}
> -
>  		ret = -ENOMEM;
>  		blob = kmalloc(params.len, GFP_KERNEL);
>  		if (!blob)
> @@ -6290,13 +6288,13 @@ static int sev_launch_measure(struct kvm *kvm, struct kvm_sev_cmd *argp)
>  		goto e_free_blob;
>  
>  	if (blob) {
> -		if (copy_to_user((void __user *)(uintptr_t)params.uaddr, blob, params.len))
> +		if (copy_to_user(p, blob, params.len))
>  			ret = -EFAULT;
>  	}
>  
>  done:
>  	params.len = data->len;
> -	if (copy_to_user((void __user *)(uintptr_t)argp->data, &params, sizeof(params)))
> +	if (copy_to_user(measure, &params, sizeof(params)))
>  		ret = -EFAULT;
>  e_free_blob:
>  	kfree(blob);
> 

Queued, thanks Brijesh and Al.

Paolo