KVM: SEV: Reject non-positive effective lengths during LAUNCH_UPDATE

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'.

Note #2, the PAGE_ALIGNED(params.len) check ensures that lengths between
1 and 4095 (inclusive) are also rejected, i.e. that KVM won't end up with
npages=0 when doing "npages = params.len / PAGE_SIZE".

Cc: Thomas Lendacky <thomas.lendacky@amd.com>
Cc: Michael Roth <michael.roth@amd.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20250919211649.1575654-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/svm/sev.c

@@ -2353,7 +2353,7 @@ static int snp_launch_update(struct kvm *kvm, struct kvm_sev_cmd *argp)
 	pr_debug("%s: GFN start 0x%llx length 0x%llx type %d flags %d\n", __func__,
 		 params.gfn_start, params.len, params.type, params.flags);
 
-	if (!PAGE_ALIGNED(params.len) || params.flags ||
+	if (!params.len || !PAGE_ALIGNED(params.len) || params.flags ||
 	    (params.type != KVM_SEV_SNP_PAGE_TYPE_NORMAL &&
 	     params.type != KVM_SEV_SNP_PAGE_TYPE_ZERO &&
 	     params.type != KVM_SEV_SNP_PAGE_TYPE_UNMEASURED &&

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);