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