linux-kernel - [PATCH 05/11] KVM: guest_memfd: limit overzealous WARN

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Date: Thu,  4 Apr 2024 14:50:27 -0400
From: Paolo Bonzini <pbonzini@...hat.com>
To: linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Cc: seanjc@...gle.com,
	michael.roth@....com,
	isaku.yamahata@...el.com
Subject: [PATCH 05/11] KVM: guest_memfd: limit overzealous WARN

Because kvm_gmem_get_pfn() is called from the page fault path without
any of the slots_lock, filemap lock or mmu_lock taken, it is
possible for it to race with kvm_gmem_unbind().  This is not a
problem, as any PTE that is installed temporarily will be zapped
before the guest has the occasion to run.

However, it is not possible to have a complete unbind+bind
racing with the page fault, because deleting the memslot
will call synchronize_srcu_expedited() and wait for the
page fault to be resolved.  Thus, we can still warn if
the file is there and is not the one we expect.

Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 virt/kvm/guest_memfd.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 409cf9b51313..e5b3cd02b651 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -499,7 +499,8 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,

 	gmem = file->private_data;

-	if (WARN_ON_ONCE(xa_load(&gmem->bindings, index) != slot)) {
+	if (xa_load(&gmem->bindings, index) != slot) {
+		WARN_ON_ONCE(xa_load(&gmem->bindings, index));
 		r = -EIO;
 		goto out_fput;
 	}
-- 
2.43.0