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

[Greg Kroah-Hartman]

On Thu, Feb 28, 2019 at 04:14:50PM +0100, Paolo Bonzini wrote:
> On 28/02/19 16:08, 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: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Cc: "Radim KrÄmÃÅ" <rkrcmar@xxxxxxxxxx>
> > Cc: kvm@xxxxxxxxxxxxxxx
> > Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > 
> > ---
> > 
> > Paolo, this should be merged into 5.0-final, and if not there, then
> > 5.1-rc1 and then backported to 5.0 through the stable tree.  If you
> > want me to send this to Linus, I will be glad to do so.
> > 
> > 
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 585845203db8..076bc38963bf 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -4044,7 +4044,7 @@ static void kvm_uevent_notify_change(unsigned int type, struct kvm *kvm)
> >  	}
> >  	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) {
> > 
> 
> Sure, go ahead.
> 
> Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

Wonderful, will do so right now, thanks!

greg k-h