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Subject[PATCH 2/4] KVM: x86/mmu: Fix RCU usage for tdp_iter_root_pt
From
The root page table in the TDP MMU paging structure is not protected
with RCU, but rather by the root_count in the associated SP. As a result
it is safe for tdp_iter_root_pt to simply return a u64 *. This sidesteps
the complexities assoicated with propagating the __rcu annotation
around.

Reported-by: kernel test robot <lkp@xxxxxxxxx>
Signed-off-by: Ben Gardon <bgardon@google.com>
---
arch/x86/kvm/mmu/tdp_iter.c | 10 ++++++++--
arch/x86/kvm/mmu/tdp_iter.h | 2 +-
arch/x86/kvm/mmu/tdp_mmu.c | 4 ++--
3 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_iter.c b/arch/x86/kvm/mmu/tdp_iter.c
index e5f148106e20..8e2c053533b6 100644
--- a/arch/x86/kvm/mmu/tdp_iter.c
+++ b/arch/x86/kvm/mmu/tdp_iter.c
@@ -159,8 +159,14 @@ void tdp_iter_next(struct tdp_iter *iter)
iter->valid = false;
}

-tdp_ptep_t tdp_iter_root_pt(struct tdp_iter *iter)
+u64 *tdp_iter_root_pt(struct tdp_iter *iter)
{
- return iter->pt_path[iter->root_level - 1];
+ /*
+ * Though it is stored in an array of tdp_ptep_t for convenience,
+ * the root PT is not actually protected by RCU, but by the root
+ * count on the associated struct kvm_mmu_page. As a result it's
+ * safe to rcu_dereference and return the value here.
+ */
+ return rcu_dereference(iter->pt_path[iter->root_level - 1]);
}

diff --git a/arch/x86/kvm/mmu/tdp_iter.h b/arch/x86/kvm/mmu/tdp_iter.h
index 4cc177d75c4a..5a47c57810ab 100644
--- a/arch/x86/kvm/mmu/tdp_iter.h
+++ b/arch/x86/kvm/mmu/tdp_iter.h
@@ -62,6 +62,6 @@ tdp_ptep_t spte_to_child_pt(u64 pte, int level);
void tdp_iter_start(struct tdp_iter *iter, u64 *root_pt, int root_level,
int min_level, gfn_t next_last_level_gfn);
void tdp_iter_next(struct tdp_iter *iter);
-tdp_ptep_t tdp_iter_root_pt(struct tdp_iter *iter);
+u64 *tdp_iter_root_pt(struct tdp_iter *iter);

#endif /* __KVM_X86_MMU_TDP_ITER_H */
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 5387ac040f66..6c8824bcc2f2 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -558,7 +558,7 @@ static inline void __tdp_mmu_set_spte(struct kvm *kvm, struct tdp_iter *iter,
u64 new_spte, bool record_acc_track,
bool record_dirty_log)
{
- tdp_ptep_t root_pt = tdp_iter_root_pt(iter);
+ u64 *root_pt = tdp_iter_root_pt(iter);
struct kvm_mmu_page *root = sptep_to_sp(root_pt);
int as_id = kvm_mmu_page_as_id(root);

@@ -653,7 +653,7 @@ static inline bool tdp_mmu_iter_cond_resched(struct kvm *kvm,

WARN_ON(iter->gfn > iter->next_last_level_gfn);

- tdp_iter_start(iter, iter->pt_path[iter->root_level - 1],
+ tdp_iter_start(iter, tdp_iter_root_pt(iter),
iter->root_level, iter->min_level,
iter->next_last_level_gfn);

--
2.31.0.rc2.261.g7f71774620-goog
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