linux-kernel - Re: [PATCH] KVM: SVM: Fix sparse: incorrect type in argument 1 (different base types)

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Message-ID: <20180221174910.GI30522@ZenIV.linux.org.uk>
Date:   Wed, 21 Feb 2018 17:49:10 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Borislav Petkov <bp@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        linux-kernel@...r.kernel.org, Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH] KVM: SVM: Fix sparse: incorrect type in argument 1
 (different base types)

On Mon, Feb 19, 2018 at 10:12:28AM -0600, Brijesh Singh wrote:
> Fix sparse: incorrect type in argument 1 (different base types). Typecast
> the userspace address argument.

Better question: why the hell do we want that access_ok(), anyway?  The only
thing we do to params.uaddr is
        if (blob) {
                if (copy_to_user((void __user *)(uintptr_t)params.uaddr, blob, params.len))
                        ret = -EFAULT;
        }

downstream.  What does that access_ok() buy us?  It does not guarantee that
copy_to_user() won't fail.  It does not clamp params.len (we'd just done
that explicitly).  So why not somethings like this:

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index b3e488a74828..ba2c1a606985 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -6239,13 +6239,15 @@ static int sev_launch_measure(struct kvm *kvm, struct kvm_sev_cmd *argp)
 	struct kvm_sev_info *sev = &kvm->arch.sev_info;
 	struct sev_data_launch_measure *data;
 	struct kvm_sev_launch_measure params;
+	void __user *measure = (void __user *)(uintptr_t)argp->data;
+	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);