[PATCH] KVM: SEV: Reject non-positive effective lengths during LAUNCH_UPDATE

[Sean Christopherson <seanjc@google.com>]

Check for an invalid length during LAUNCH_UPDATE at the start of
snp_launch_update() instead of subtly relying on kvm_gmem_populate() to
detect the bad state.  Code that directly handles userspace input
absolutely should sanitize those inputs; failure to do so is asking for
bugs where KVM consumes an invalid "npages".

Keep the check in gmem, but wrap it in a WARN to flag any bad usage by
the caller.

Note, this is technically an ABI change as KVM would previously allow a
length of '0'.  But allowing a length of '0' is nonsensical and creates
pointless conundrums in KVM.  E.g. an empty range is arguably neither
private nor shared, but LAUNCH_UPDATE will fail if the starting gpa can't
be made private.  In practice, no known or well-behaved VMM passes a
length of '0'.

Cc: Thomas Lendacky <thomas.lendacky@amd.com>
Cc: Michael Roth <michael.roth@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---

Compile tested only.  Came across this when trying to figure out how to
handle the batching of gmem post-populate calls.

 arch/x86/kvm/svm/sev.c | 2 ++
 virt/kvm/guest_memfd.c | 3 ++-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index f4381878a9e5..746a57bf1f71 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2360,6 +2360,8 @@ static int snp_launch_update(struct kvm *kvm, struct kvm_sev_cmd *argp)
 		return -EINVAL;
 
 	npages = params.len / PAGE_SIZE;
+	if (npages <= 0)
+		return -EINVAL;
 
 	/*
 	 * For each GFN that's being prepared as part of the initial guest
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 7d85cc33c0bb..79552467add5 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -639,7 +639,8 @@ long kvm_gmem_populate(struct kvm *kvm, gfn_t start_gfn, void __user *src, long
 	long i;
 
 	lockdep_assert_held(&kvm->slots_lock);
-	if (npages < 0)
+
+	if (WARN_ON_ONCE(npages <= 0))
 		return -EINVAL;
 
 	slot = gfn_to_memslot(kvm, start_gfn);

base-commit: ecbcc2461839e848970468b44db32282e5059925
-- 
2.51.0.268.g9569e192d0-goog