Re: [PATCH] kvm: properly check debugfs dentry before using it

[Eric Biggers]

On Thu, Feb 28, 2019 at 04:34:37PM +0100, Greg Kroah-Hartman wrote:
> debugfs can now report an error code if something went wrong instead of
> just NULL.  So if the return value is to be used as a "real" dentry, it
> needs to be checked if it is an error before dereferencing it.
> 
> This is now happening because of ff9fb72bc077 ("debugfs: return error
> values, not NULL").  syzbot has found a way to trigger multiple debugfs
> files attempting to be created, which fails, and then the error code
> gets passed to dentry_path_raw() which obviously does not like it.
> 
> Reported-by: Eric Biggers <ebiggers@xxxxxxxxxx>
> Reported-and-tested-by: syzbot+7857962b4d45e602b8ad@xxxxxxxxxxxxxxxxxxxxxxxxx
> Cc: "Radim KrÄmÃÅ" <rkrcmar@xxxxxxxxxx>
> Cc: kvm@xxxxxxxxxxxxxxx
> Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> ---
> 
> Linus, this should go in before 5.0-final is out, as it resolves a
> problem found by syzbot.  Paolo has given his ack for me to send this
> directly to you.  If you want this in [GIT PULL] format, I can do that
> as well.
> 
>  virt/kvm/kvm_main.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -4044,7 +4044,7 @@ static void kvm_uevent_notify_change(uns
>  	}
>  	add_uevent_var(env, "PID=%d", kvm->userspace_pid);
>  
> -	if (kvm->debugfs_dentry) {
> +	if (!IS_ERR_OR_NULL(kvm->debugfs_dentry)) {
>  		char *tmp, *p = kmalloc(PATH_MAX, GFP_KERNEL);
>  
>  		if (p) {

So what about the other checks of kvm->debugfs_dentry, in
kvm_destroy_vm_debugfs() and kvm_create_vcpu_debugfs()?

- Eric