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

On 8/26/25 18:37, Sean Christopherson wrote:
> 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;

Would it make sense to include a !params.len in the giant if check just
above this, e.g.:

	if (!params.len || !PAGE_ALIGNED(params.len) || ...

?

That way everything related to checking "params" remains in the one
statement.

Thanks,
Tom

>  
>  	/*
>  	 * 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

On Mon, Sep 08, 2025, Tom Lendacky wrote:
> On 8/26/25 18:37, Sean Christopherson wrote:
> > 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;
> 
> Would it make sense to include a !params.len in the giant if check just
> above this, e.g.:
> 
> 	if (!params.len || !PAGE_ALIGNED(params.len) || ...
> 
> ?
> 
> That way everything related to checking "params" remains in the one
> statement.

Oh, yeah, duh.  I overlooked that the only way for npages to be '0' is if
params.len is '0', because the PAGE_ALIGNED() check will handed len == 1-4095.

Will send a v2.  Thanks Tom!