When clearing TDP MMU pages what have been disconnected from the paging
structure root, set the SPTEs to a special non-present value which will
not be overwritten by other threads. This is needed to prevent races in
which a thread is clearing a disconnected page table, but another thread
has already acquired a pointer to that memory and installs a mapping in
an already cleared entry. This can lead to memory leaks and accounting
errors.
Reviewed-by: Peter Feiner <pfeiner@xxxxxxxxxx>
Signed-off-by: Ben Gardon <bgardon@xxxxxxxxxx>
---
arch/x86/kvm/mmu/tdp_mmu.c | 36 ++++++++++++++++++++++++++++++------
1 file changed, 30 insertions(+), 6 deletions(-)
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 7a2cdfeac4d2..0dd27e000dd0 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -334,9 +334,10 @@ static void handle_removed_tdp_mmu_page(struct kvm *kvm, u64 *pt,
{
struct kvm_mmu_page *sp = sptep_to_sp(pt);
int level = sp->role.level;
- gfn_t gfn = sp->gfn;
+ gfn_t base_gfn = sp->gfn;
u64 old_child_spte;
u64 *sptep;
+ gfn_t gfn;
int i;
trace_kvm_mmu_prepare_zap_page(sp);
@@ -345,16 +346,39 @@ static void handle_removed_tdp_mmu_page(struct kvm *kvm, u64 *pt,
for (i = 0; i < PT64_ENT_PER_PAGE; i++) {
sptep = pt + i;
+ gfn = base_gfn + (i * KVM_PAGES_PER_HPAGE(level - 1));
if (shared) {
- old_child_spte = xchg(sptep, 0);
+ /*
+ * Set the SPTE to a nonpresent value that other
+ * threads will not overwrite. If the SPTE was
+ * already marked as removed then another thread
+ * handling a page fault could overwrite it, so
+ * set the SPTE until it is set from some other
+ * value to the removed SPTE value.
+ */
+ for (;;) {
+ old_child_spte = xchg(sptep, REMOVED_SPTE);
+ if (!is_removed_spte(old_child_spte))
+ break;
+ cpu_relax();
+ }
} else {
old_child_spte = READ_ONCE(*sptep);
- WRITE_ONCE(*sptep, 0);
+
+ /*
+ * Marking the SPTE as a removed SPTE is not
+ * strictly necessary here as the MMU lock should
+ * stop other threads from concurrentrly modifying
+ * this SPTE. Using the removed SPTE value keeps
+ * the shared and non-atomic cases consistent and
+ * simplifies the function.
+ */
+ WRITE_ONCE(*sptep, REMOVED_SPTE);
}
- handle_changed_spte(kvm, kvm_mmu_page_as_id(sp),
- gfn + (i * KVM_PAGES_PER_HPAGE(level - 1)),
- old_child_spte, 0, level - 1, shared);
+ handle_changed_spte(kvm, kvm_mmu_page_as_id(sp), gfn,
+ old_child_spte, REMOVED_SPTE, level - 1,
+ shared);
}
kvm_flush_remote_tlbs_with_address(kvm, gfn,
--
2.30.0.365.g02bc693789-goog
