Re: [PATCH] KVM: Do not leak memory for duplicate debugfs directories

[Greg Kroah-Hartman]

On Wed, Aug 04, 2021 at 05:37:37AM -0400, Paolo Bonzini wrote:
> KVM creates a debugfs directory for each VM in order to store statistics
> about the virtual machine.  The directory name is built from the process
> pid and a VM fd.  While generally unique, it is possible to keep a
> file descriptor alive in a way that causes duplicate directories, which
> manifests as these messages:
> 
>   [  471.846235] debugfs: Directory '20245-4' with parent 'kvm' already present!
> 
> Even though this should not happen in practice, it is more or less
> expected in the case of KVM for testcases that call KVM_CREATE_VM and
> close the resulting file descriptor repeatedly and in parallel.
> 
> When this happens, debugfs_create_dir() returns an error but
> kvm_create_vm_debugfs() goes on to allocate stat data structs which are
> later leaked.  The slow memory leak was spotted by syzkaller, where it
> caused OOM reports.
> 
> Since the issue only affects debugfs, do a lookup before calling
> debugfs_create_dir, so that the message is downgraded and rate-limited.
> While at it, ensure kvm->debugfs_dentry is NULL rather than an error
> if it is not created.  This fixes kvm_destroy_vm_debugfs, which was not
> checking IS_ERR_OR_NULL correctly.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 536a6f88c49d ("KVM: Create debugfs dir and stat files for each VM")
> Reported-by: Alexey Kardashevskiy <aik@xxxxxxxxx>
> Suggested-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
>  virt/kvm/kvm_main.c | 18 ++++++++++++++++--
>  1 file changed, 16 insertions(+), 2 deletions(-)

Acked-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>