[PATCH v2 1/2] KVM: SEV: Explicitly disallow NULL user address for SNP_LAUNCH_UPDATE

[Sean Christopherson <seanjc@google.com>]

From: Joerg Roedel <joerg.roedel@amd.com>

Explicitly reject a NULL userspace virtual address for the source page of
SNP_LAUNCH_UPDATE instead of relying on the post-populate callback to do
the check, and don't WARN on failure, as the scenario is blatantly user-
triggerable, as reported by Sashiko.  Waiting until post-populate to check
the address "works", but makes it unnecessarily difficult to see that KVM's
ABI is to disallow a NULL source page for non-ZERO pages.

Note, several existing VMMs pass a valid userspace address for the ZERO
case, i.e. KVM can't *require* the userspace address to be NULL for ZERO
pages, at least not without breaking userspace.

Fixes: dee5a47cc7a4 ("KVM: SEV: Add KVM_SEV_SNP_LAUNCH_UPDATE command")
Reported-by: Sashiko Bot <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/all/20260611125849.9ED631F00893@smtp.kernel.org
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/svm/sev.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 74fb15551e83..621a2eaa58f2 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2330,9 +2330,6 @@ static int sev_gmem_post_populate(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
 	int level;
 	int ret;
 
-	if (WARN_ON_ONCE(sev_populate_args->type != KVM_SEV_SNP_PAGE_TYPE_ZERO && !src_page))
-		return -EINVAL;
-
 	ret = snp_lookup_rmpentry((u64)pfn, &assigned, &level);
 	if (ret || assigned) {
 		pr_debug("%s: Failed to ensure GFN 0x%llx RMP entry is initial shared state, ret: %d assigned: %d\n",
@@ -2421,10 +2418,12 @@ static int snp_launch_update(struct kvm *kvm, struct kvm_sev_cmd *argp)
 	     params.type != KVM_SEV_SNP_PAGE_TYPE_CPUID))
 		return -EINVAL;
 
-	src = params.type == KVM_SEV_SNP_PAGE_TYPE_ZERO ? NULL : u64_to_user_ptr(params.uaddr);
-
-	if (!PAGE_ALIGNED(src))
+	if (params.type == KVM_SEV_SNP_PAGE_TYPE_ZERO)
+		src = NULL;
+	else if (!params.uaddr || !PAGE_ALIGNED(params.uaddr))
 		return -EINVAL;
+	else
+		src = u64_to_user_ptr(params.uaddr);
 
 	npages = params.len / PAGE_SIZE;
 
-- 
2.55.0.rc0.799.gd6f94ed593-goog