linux-kernel - [RFC PATCH kernel] KVM: Stop leaking memory in debugfs

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Date:   Fri, 30 Jul 2021 14:32:17 +1000
From:   Alexey Kardashevskiy <aik@...abs.ru>
To:     linux-kernel@...r.kernel.org
Cc:     Alexey Kardashevskiy <aik@...abs.ru>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: [RFC PATCH kernel] KVM: Stop leaking memory in debugfs

The debugfs folder name is made of a supposedly unique pair of
the process pid and a VM fd. However it is possible to get a race here
which manifests in these messages:

[  471.846235] debugfs: Directory '20245-4' with parent 'kvm' already present!

debugfs_create_dir() returns an error which is handled correctly
everywhere except kvm_create_vm_debugfs() where the code allocates
stat data structs and overwrites the older values regardless.

Spotted by syzkaller. This slow memory leak produces way too many
OOM reports.

Signed-off-by: Alexey Kardashevskiy <aik@...abs.ru>
---

I am pretty sure we better fix the race but I am not quite sure what
lock is appropriate here, ideas?

---
 virt/kvm/kvm_main.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 986959833d70..89496fd8127a 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -904,6 +904,10 @@ static int kvm_create_vm_debugfs(struct kvm *kvm, int fd)

 	snprintf(dir_name, sizeof(dir_name), "%d-%d", task_pid_nr(current), fd);
 	kvm->debugfs_dentry = debugfs_create_dir(dir_name, kvm_debugfs_dir);
+	if (IS_ERR_OR_NULL(kvm->debugfs_dentry)) {
+		pr_err("Failed to create %s\n", dir_name);
+		return 0;
+	}

 	kvm->debugfs_stat_data = kcalloc(kvm_debugfs_num_entries,
 					 sizeof(*kvm->debugfs_stat_data),
-- 
2.30.2