[PATCH 05/13] KVM: Fix MMU invalidation bookkeeping in guest_memfd

Sean Christopherson
2023-09-21T20:56:36Z

Acquire mmu_lock and do invalidate_{begin,end}() if and only if there is
at least one memslot that overlaps the to-be-invalidated range. This
fixes a bug where KVM would leave a danging in-progress invalidation as
the begin() call was unconditional, but the end() was not (only performed
if there was overlap).

Reported-by: Binbin Wu <binbin.wu@linux.intel.com>
Fixes: 1d46f95498c5 ("KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory")
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
virt/kvm/guest_mem.c | 23 ++++++++++++++---------
1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/virt/kvm/guest_mem.c b/virt/kvm/guest_mem.c
index 3c9e83a596fe..68528e9cddd7 100644
--- a/virt/kvm/guest_mem.c
+++ b/virt/kvm/guest_mem.c
@@ -88,14 +88,10 @@ static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index)
static void kvm_gmem_invalidate_begin(struct kvm_gmem *gmem, pgoff_t start,
pgoff_t end)
{
+ bool flush = false, found_memslot = false;
struct kvm_memory_slot *slot;
struct kvm *kvm = gmem->kvm;
unsigned long index;
- bool flush = false;
-
- KVM_MMU_LOCK(kvm);
-
- kvm_mmu_invalidate_begin(kvm);

xa_for_each_range(&gmem->bindings, index, slot, start, end - 1) {
pgoff_t pgoff = slot->gmem.pgoff;
@@ -107,13 +103,21 @@ static void kvm_gmem_invalidate_begin(struct kvm_gmem *gmem, pgoff_t start,
.may_block = true,
};

+ if (!found_memslot) {
+ found_memslot = true;
+
+ KVM_MMU_LOCK(kvm);
+ kvm_mmu_invalidate_begin(kvm);
+ }
+
flush |= kvm_mmu_unmap_gfn_range(kvm, &gfn_range);
}

if (flush)
kvm_flush_remote_tlbs(kvm);

- KVM_MMU_UNLOCK(kvm);
+ if (found_memslot)
+ KVM_MMU_UNLOCK(kvm);
}

static void kvm_gmem_invalidate_end(struct kvm_gmem *gmem, pgoff_t start,
@@ -121,10 +125,11 @@ static void kvm_gmem_invalidate_end(struct kvm_gmem *gmem, pgoff_t start,
{
struct kvm *kvm = gmem->kvm;

- KVM_MMU_LOCK(kvm);
- if (xa_find(&gmem->bindings, &start, end - 1, XA_PRESENT))
+ if (xa_find(&gmem->bindings, &start, end - 1, XA_PRESENT)) {
+ KVM_MMU_LOCK(kvm);
kvm_mmu_invalidate_end(kvm);
- KVM_MMU_UNLOCK(kvm);
+ KVM_MMU_UNLOCK(kvm);
+ }
}

static long kvm_gmem_punch_hole(struct inode *inode, loff_t offset, loff_t len)
--
2.42.0.515.g380fc7ccd1-goog