[PATCH v2] KVM: x86: Take SRCU in kvm_zap_gfn_range()

kvm_zap_gfn_range() walks memslots and rmaps and may drop mmu_lock to
reschedule. Callers that do not already hold kvm->srcu (or slots_lock)
can race with memslot deletion: synchronize_srcu_expedited() does not
wait, kvm_free_memslot() frees the old slot and its rmap, and the zap
resumes on freed memory.

The VFIO noncoherent-DMA path hits this by zapping the entire GPA
space without SRCU. KASAN reported:

  BUG: KASAN: vmalloc-out-of-bounds in slot_rmap_walk_next+0x82/0x1c0
  Read of size 8 at addr ffffc900005c1008
  Call Trace:
   slot_rmap_walk_next+0x82/0x1c0
   __kvm_rmap_zap_gfn_range+0x17a/0x280
   kvm_zap_gfn_range+0x2a6/0x6a0
   kvm_vfio_set_attr+0x576/0x770
   kvm_device_ioctl+0x1ff/0x3b0

Take SRCU inside kvm_zap_gfn_range() so every caller is covered.
Nesting with an existing kvm->srcu critical section is fine; the
helper uses a local index. Drop the now-redundant SRCU pair from
__kvm_set_or_clear_apicv_inhibit().

Fixes: 362ff6dca541 ("KVM: x86/mmu: Zap KVM TDP when noncoherent DMA assignment starts/stops")
Cc: [email protected]
Signed-off-by: Chengfeng Ye <[email protected]>
---
v2:
- Take SRCU inside kvm_zap_gfn_range() with guard(srcu)(), as suggested
  by Sean Christopherson, instead of wrapping only the noncoherent-DMA
  caller.
- Drop the now-redundant SRCU pair from
  __kvm_set_or_clear_apicv_inhibit(). Nested kvm->srcu is fine; only
  kvm_vcpu_srcu_read_lock() cannot nest with itself.

 arch/x86/kvm/mmu/mmu.c | 2 ++
 arch/x86/kvm/x86.c     | 2 --
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index a61750f8e1e3..ae55a77e5a05 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -7047,6 +7047,8 @@ void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end)
 	if (WARN_ON_ONCE(gfn_end <= gfn_start))
 		return;
 
+	guard(srcu)(&kvm->srcu);
+
 	write_lock(&kvm->mmu_lock);
 
 	kvm_mmu_invalidate_start(kvm);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 69469bbdc84a..adccd4a8e6c1 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10986,10 +10986,8 @@ void __kvm_set_or_clear_apicv_inhibit(struct kvm *kvm,
 		kvm->arch.apicv_inhibit_reasons = new;
 		if (new) {
 			unsigned long gfn = gpa_to_gfn(APIC_DEFAULT_PHYS_BASE);
-			int idx = srcu_read_lock(&kvm->srcu);
 
 			kvm_zap_gfn_range(kvm, gfn, gfn+1);
-			srcu_read_unlock(&kvm->srcu, idx);
 		}
 	} else {
 		kvm->arch.apicv_inhibit_reasons = new;
-- 
2.43.0