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

[Greg Kroah-Hartman]

On Thu, Feb 28, 2019 at 09:17:28AM -0800, Eric Biggers wrote:
> 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()?

Those are fine, they do not matter as they are only calling other
debugfs calls which can handle error codes just fine.  The only issue is
when you try to use the dentry for something "real" like this that bad
things can happen.

Luckily this was only the case in about 4 places in the kernel, all of
which should be fixed now.

thanks,

greg k-h