[PATCH 09/12] KVM: guest_memfd: move check for already-populated page to common code

Paolo Bonzini
2024-07-11T22:28:08Z

Do not allow populating the same page twice with startup data. In the
case of SEV-SNP, for example, the firmware does not allow it anyway,
since the launch-update operation is only possible on pages that are
still shared in the RMP.

Even if it worked, kvm_gmem_populate()'s callback is meant to have side
effects such as updating launch measurements, and updating the same
page twice is unlikely to have the desired results.

Races between calls to the ioctl are not possible because kvm_gmem_populate()
holds slots_lock and the VM should not be running. But again, even if
this worked on other confidential computing technology, it doesn't matter
to guest_memfd.c whether this is an intentional attempt to do something
fishy, or missing synchronization in userspace, or even something
intentional. One of the racers wins, and the page is initialized by
either kvm_gmem_prepare_folio() or kvm_gmem_populate().

Anyway, out of paranoia, adjust sev_gmem_post_populate() anyway to use
the same errno that kvm_gmem_populate() is using.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
arch/x86/kvm/svm/sev.c | 2 +-
virt/kvm/guest_memfd.c | 7 +++++++
2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index df8818759698..397ef9e70182 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2213,7 +2213,7 @@ static int sev_gmem_post_populate(struct kvm *kvm, gfn_t gfn_start, kvm_pfn_t pf
if (ret || assigned) {
pr_debug("%s: Failed to ensure GFN 0x%llx RMP entry is initial shared state, ret: %d assigned: %d\n",
__func__, gfn, ret, assigned);
- ret = -EINVAL;
+ ret = ret ? -EINVAL : -EEXIST;
goto err;
}

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 509360eefea5..266810bb91c9 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -650,6 +650,13 @@ long kvm_gmem_populate(struct kvm *kvm, gfn_t start_gfn, void __user *src, long
break;
}

+ if (folio_test_uptodate(folio)) {
+ folio_unlock(folio);
+ folio_put(folio);
+ ret = -EEXIST;
+ break;
+ }
+
folio_unlock(folio);
if (!IS_ALIGNED(gfn, (1 << max_order)) ||
(npages - i) < (1 << max_order))
--
2.43.0