Received-SPF: pass (google.com: domain of linux-kernel+bounces-171956-linux.lists.archive=gmail.com@vger.kernel.org designates 139.178.88.99 as permitted sender) client-ip=139.178.88.99;
From: Paolo Bonzini <pbonzini@redhat.com>
To: linux-kernel@vger.kernel.org,
	kvm@vger.kernel.org
Cc: vbabka@suse.cz,
	isaku.yamahata@intel.com,
	xiaoyao.li@intel.com,
	binbin.wu@linux.intel.com,
	seanjc@google.com,
	rick.p.edgecombe@intel.com,
	michael.roth@amd.com,
	yilun.xu@intel.com
Subject: [PATCH 4/9] KVM: guest_memfd: limit overzealous WARN
Date: Tue,  7 May 2024 14:07:24 -0400
Message-ID: <20240507180729.3975856-5-pbonzini@redhat.com>
In-Reply-To: <20240507180729.3975856-1-pbonzini@redhat.com>
References: <20240507180729.3975856-1-pbonzini@redhat.com>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain
Content-Transfer-Encoding: 8bit

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@redhat.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 c27828b0d42d..fd32288d0fbc 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