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 Cc: Michael Roth Signed-off-by: Sean Christopherson --- v2: Check params.len right away. [Tom] v1: https://lore.kernel.org/all/20250826233734.4011090-1-seanjc@google.com arch/x86/kvm/svm/sev.c | 2 +- virt/kvm/guest_memfd.c | 3 ++- 2 files changed, 3 insertions(+), 2 deletions(-) diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c index cce48fff2e6c..31b3e128e521 100644 --- a/arch/x86/kvm/svm/sev.c +++ b/arch/x86/kvm/svm/sev.c @@ -2370,7 +2370,7 @@ 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 && diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c index 08a6bc7d25b6..1d323ca178cb 100644 --- a/virt/kvm/guest_memfd.c +++ b/virt/kvm/guest_memfd.c @@ -716,7 +716,8 @@ 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: c8fbf7ceb2ae3f64b0c377c8c21f6df577a13eb4 -- 2.51.0.470.ga7dc726c21-goog