[PATCH v3 24/28] KVM: x86/mmu: WARN on any attempt to atomically update REMOVED SPTE

Sean Christopherson
2022-02-26T00:18:32Z

Disallow calling tdp_mmu_set_spte_atomic() with a REMOVED "old" SPTE.
This solves a conundrum introduced by commit 3255530ab191 ("KVM: x86/mmu:
Automatically update iter->old_spte if cmpxchg fails"); if the helper
doesn't update old_spte in the REMOVED case, then theoretically the
caller could get stuck in an infinite loop as it will fail indefinitely
on the REMOVED SPTE. E.g. until recently, clear_dirty_gfn_range() didn't
check for a present SPTE and would have spun until getting rescheduled.

In practice, only the page fault path should "create" a new SPTE, all
other paths should only operate on existing, a.k.a. shadow present,
SPTEs. Now that the page fault path pre-checks for a REMOVED SPTE in all
cases, require all other paths to indirectly pre-check by verifying the
target SPTE is a shadow-present SPTE.

Note, this does not guarantee the actual SPTE isn't REMOVED, nor is that
scenario disallowed. The invariant is only that the caller mustn't
invoke tdp_mmu_set_spte_atomic() if the SPTE was REMOVED when last
observed by the caller.

Cc: David Matlack <dmatlack@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/mmu/tdp_mmu.c | 15 +++++++--------
1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 1acd12bf309f..d223870b3790 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -634,16 +634,15 @@ static inline int tdp_mmu_set_spte_atomic(struct kvm *kvm,
u64 *sptep = rcu_dereference(iter->sptep);
u64 old_spte;

- WARN_ON_ONCE(iter->yielded);
-
- lockdep_assert_held_read(&kvm->mmu_lock);
-
/*
- * Do not change removed SPTEs. Only the thread that froze the SPTE
- * may modify it.
+ * The caller is responsible for ensuring the old SPTE is not a REMOVED
+ * SPTE. KVM should never attempt to zap or manipulate a REMOVED SPTE,
+ * and pre-checking before inserting a new SPTE is advantageous as it
+ * avoids unnecessary work.
*/
- if (is_removed_spte(iter->old_spte))
- return -EBUSY;
+ WARN_ON_ONCE(iter->yielded || is_removed_spte(iter->old_spte));
+
+ lockdep_assert_held_read(&kvm->mmu_lock);

/*
* Note, fast_pf_fix_direct_spte() can also modify TDP MMU SPTEs and
--
2.35.1.574.g5d30c73bfb-goog